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Thread: WITH RECURSIVE patch V0.1

WITH RECURSIVE patch V0.1

From
Tatsuo Ishii
Date:
WITH RECURSIVE patch V0.1

Here are patches to implement WITH RECURSIVE clause. There are some
limitiations and TODO items(see the "Current limitations" section
below). Comments are welcome.

1. Credit

These patches were developed by Yoshiyuki Asaba (y-asab@sraoss.co.jp)
with some discussions with Tatsuo Ishii (ishii@sraoss.co.jp).

- prior patches:

http://archives.postgresql.org/pgsql-patches/2008-03/msg00327.php

The patches include some extentions to the patches above.

2. Design proposales

See: http://archives.postgresql.org/pgsql-hackers/2008-02/msg00642.php

3. Implementation details

- Parsing recursive queries

We convert WITH clause to following structure in analyzer.

/*
 * WithClause -
 *     reprensentation of WITH clause
 */
typedef struct WithClause
{
    NodeTag        type;
    List        *subquery;        /* query list */
    bool        recursive;        /* true = WITH RECURSIVE */
} WithClause;

If recursive is false, WITH clause is converted to a subquery.

Next we call transformWithClause() to check if the query defined in
WITH RECURSIVE clause is a liner recursive query.

transformWithClause() does followings:

1) Detect query which is not allowed in SQL standard

2) Create a name dependency graphs

transoform* functions checks if target tables actually exist and throw
an error if tables do not exist.

For example in following query:

WITH RECURSIVE x AS (SELECT * from y),
                y AS (SELECT * FROM pg_class)
  SELECT * FROM x;

x's definition referes to y and y is defined after x's definition. If
we evaluate x and y in the order of above, an error will occur while
evaluating x since y is not defined yet.

To avoid the problem we create new function:

void checkWellFormedCte(ParseState *pstate, WithClause *withClause);

which performs topological sort on graphs.

Non recursive term will be the start of the graph, checks type
checking one by one.

names appear in WITH RECURSIVE clause form a name dependency
graph. The nodes in the graph is represented by CteNode type:

typedef struct CteNode;
struct CteNode
{
    Alias *alias;
    CteList *dependency; /* if NULL the node does not call other node */
    bool self_reference;
    bool non_recursive;
} CteNode;

If there is loop in the dependency graph, a matual recursion exists
and an error occurs (error code 0A00).
* Note that this part is not implemented in the V1.0 patches yet.

Tables defined in WITH RECURSIVE clause is identified as RTE_RECURSIVE.
A name space is added to p_recursive_namespace in ParseState
structure. The added item's structure is following:

typedef struct RangeRecursive
{
    NodeTag        type;
    Node       *subquery; /* whole subquery */
    Alias       *alias;            /* table alias & optional column aliases */
    List       *coldeflist;        /* list of ColumnDef nodes to describe result
                                 * of function returning RECORD */
    Node        *non_recursive_term; /* anaylze result of non recursive term */
    bool        recursive; /* reserved */
} RangeRecursive;

non_recursive_term keeps the result of analyze on non recursive term.
Using this, the type of recursive query is inferenced.

- Generating a plan

New plan node "Recursion" and "Recursive scan" is added.
Recursion scan is a plan which returns WT. Recursion is a plan which
execute recursive query. The subplan of Recursion is always "Append".

Below is an example EXPLAIN output.

EXPLAIN WITH RECURSIVE subdepartment AS
(
  -- non recursive term
  SELECT * FROM department WHERE name = 'A'

  UNION ALL

  -- recursive term
  SELECT d.* FROM department AS d, subdepartment
    WHERE d.parent_department = subdepartment.id
)
SELECT * FROM subdepartment;

                                        QUERY PLAN
-------------------------------------------------------------------------------------------
 Recursion on subdepartment  (cost=0.00..50.76 rows=12 width=40)
   ->  Append  (cost=0.00..50.64 rows=12 width=40)
         ->  Seq Scan on department  (cost=0.00..24.50 rows=6 width=40) <-- non recusrive term
               Filter: (name = 'A'::text)
         ->  Hash Join  (cost=0.01..26.02 rows=6 width=40)              <-- recursive term
               Hash Cond: (d.parent_department = subdepartment.id)
               ->  Seq Scan on department d  (cost=0.00..21.60 rows=1160 width=40)
               ->  Hash  (cost=0.00..0.00 rows=1 width=4)
                     ->  Recursive Scan on subdepartment  (cost=0.00..0.00 rows=1 width=4)


- Executor

new files nodeRecursion.c and nodeRecursivescan.c are added.

The structure which represents the Recursion plan is as follows:

typedef struct Recursion
{
    Scan        scan;
    Plan        *subplan;
    List       *subrtable;        /* temporary workspace for planner */
} Recursion;


typedef struct RecursionState
{
    ScanState    ss;                /* its first field is NodeTag */
    PlanState  *subplan;
    bool recursive_empty; /* if recursive term does exist, true */
    Tuplestorestate *intermediate_tuplestorestate; /* intermediate table */
    bool init_done; /* initialization flag */
} RecursiveState;

Following function are defined.

TupleTableSlot *
ExecRecursion(RecursionState *)
{
...
  if (non recursive term)
  {
      slot = ExecProcNode(non recurisve term)
      tuplestore_puttuple(WT, slot);
      return slot;
  }

  /* calculate recursive term */
  slot = ExecProcNode(recursive term)
  if (!TupIsNull(slot)
  {
      slot = ExecProcNode(Nested Loop);
      if (slot is empty)
            swap(intermediate_tuplestorestate, WT)
      else
          tuplestore_puttuple(intermediate_tuplestorestate, slot)
  }
...
}


- Executing RecursiveScan

Returns tuples store in a work table. Work table is stored in Estate structure.

typedef struct EState
{
...
    Tuplestorestate *es_tuplestorestate; /* Stuff used for recursive query */
...
} EState;

RecursiveScan and RecursiveScanState structures.

typedef struct RecursiveScan
{
    Plan       *plan;
    Plan        *subplan;
} RecursiveScan;

typedef struct RecursiveScanState
{
    ScanState     ss;
    TupleDesc    tupdesc;
    bool        done_init;
} RecursiveScanState;

RecursiveScanState initialized RecursionState the initialize the type
information. Since Recursive plan is an upper plan of RecursiveScan,
initialization should be delayed. For this purpose we use done_init
flag.

Scan functions:

static TupleTableSlot *
RecursivescanNext(RecursiveScanState *node)
{
...
    if (tuplestore_gettupleslot(node->ss.ps->state->es_tuplestorestate, true, slot))
        return slot;
...
}

TupleTableSlot *
ExecRecursiveScan(RecursiveScanState *)
{
    /*
     * Delay initializing type info because type inference did not do
     * in ExecInitRecursiveScan.
     */
    if (!node->init_done)
    {
        ExecAssignScanType(&node->ss,
                           node->ss.ps.state->es_rscan_tupledesc);

        /*
         * Initialize result tuple type and projection info.
         */
        ExecAssignResultTypeFromTL(&node->ss.ps);
        ExecAssignScanProjectionInfo(&node->ss);
        node->init_done = true;
    }
    return ExecScan(&node->ss, (ExecScanAccessMtd) RecursivescanNext);
}

4. New source codes:

- src/backend/parser/parse_cte.c
 parse CTE (not completed)

- src/backend/executor/nodeRecursion.c
- src/include/nodeRecursion.h
 Execute Recursion plan

- src/backend/executor/nodeRecursivescan.c
- src/include/nodeRecursivescan.h
 Execute Recursive scan plan

5. Limitations and TODO

1) Only the last SELECT of UNION ALL can include self recursion name.

2) Cost of Recursion and Recursivescan plan are always 0.

3) Some recursion type checking is not complete.

4) Outer join for recursive name and tables does not work.

5) Need regression test cases.

6) Documentation needed.

6. Executing WITH RECURSIVE cluase examples

DROP TABLE IF EXISTS department;
DROP TABLE
CREATE TABLE department (
  id INT PRIMARY KEY,  -- department ID
  parent_department INT REFERENCES department, -- upper department ID
  name TEXT -- department name
);
psql:recursive-e.sql:6: NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "department_pkey" for table
"department"
CREATE TABLE
INSERT INTO department VALUES (0, NULL, 'ROOT');
INSERT 0 1
INSERT INTO department VALUES (1, 0, 'A');
INSERT 0 1
INSERT INTO department VALUES (2, 1, 'B');
INSERT 0 1
INSERT INTO department VALUES (3, 2, 'C');
INSERT 0 1
INSERT INTO department VALUES (4, 2, 'D');
INSERT 0 1
INSERT INTO department VALUES (5, 0, 'E');
INSERT 0 1
INSERT INTO department VALUES (6, 4, 'F');
INSERT 0 1
INSERT INTO department VALUES (7, 5, 'G');
INSERT 0 1
-- department structure represented here is as follows:
--
-- ROOT-+->A-+->B-+->C
--      |         |
--      |         +->D-+->F
--      +->E-+->G
EXPLAIN WITH RECURSIVE subdepartment AS
(
  -- non recursive term
  SELECT * FROM department WHERE name = 'A'
  UNION ALL
  -- recursive term
  SELECT d.* FROM department AS d, subdepartment
    WHERE d.parent_department = subdepartment.id
)
SELECT * FROM subdepartment;
                                        QUERY PLAN
-------------------------------------------------------------------------------------------
 Recursion on subdepartment  (cost=0.00..50.76 rows=12 width=40)
   ->  Append  (cost=0.00..50.64 rows=12 width=40)
         ->  Seq Scan on department  (cost=0.00..24.50 rows=6 width=40)
               Filter: (name = 'A'::text)
         ->  Hash Join  (cost=0.01..26.02 rows=6 width=40)
               Hash Cond: (d.parent_department = subdepartment.id)
               ->  Seq Scan on department d  (cost=0.00..21.60 rows=1160 width=40)
               ->  Hash  (cost=0.00..0.00 rows=1 width=4)
                     ->  Recursive Scan on subdepartment  (cost=0.00..0.00 rows=1 width=4)
(9 rows)

-- extract all departments under 'A'. Result should be A, B, C, D and F
WITH RECURSIVE subdepartment AS
(
  -- non recursive term
  SELECT * FROM department WHERE name = 'A'
  UNION ALL
  -- recursive term
  SELECT d.* FROM department AS d, subdepartment
    WHERE d.parent_department = subdepartment.id
)
SELECT * FROM subdepartment ORDER BY name;
 id | parent_department | name
----+-------------------+------
  1 |                 0 | A
  2 |                 1 | B
  3 |                 2 | C
  4 |                 2 | D
  6 |                 4 | F
(5 rows)

-- extract all departments under 'E'. Result should be E and G.
WITH RECURSIVE subdepartment AS
(
  -- non recursive term
  SELECT * FROM department WHERE name = 'E'
  UNION ALL
  -- recursive term
  SELECT d.* FROM department AS d, subdepartment
    WHERE d.parent_department = subdepartment.id
)
SELECT * FROM subdepartment ORDER BY name;
 id | parent_department | name
----+-------------------+------
  5 |                 0 | E
  7 |                 5 | G
(2 rows)
--
Tatsuo Ishii
SRA OSS, Inc. Japan
Attachment

Re: WITH RECURSIVE patch V0.1

From
David Fetter
Date:
On Sun, May 18, 2008 at 08:51:29PM +0900, Tatsuo Ishii wrote:
> WITH RECURSIVE patch V0.1
>
> Here are patches to implement WITH RECURSIVE clause. There are some
> limitiations and TODO items(see the "Current limitations" section
> below). Comments are welcome.
>
> 1. Credit
>
> These patches were developed by Yoshiyuki Asaba (y-asab@sraoss.co.jp)
> with some discussions with Tatsuo Ishii (ishii@sraoss.co.jp).

This is really great!  Kudos to all who made this happen :)

I tried a bunch of different queries, and so far, only these two
haven't worked.  Any ideas what I'm doing wrong here?

WITH RECURSIVE t(n) AS (
    SELECT 1
UNION ALL
    SELECT n+1
    FROM t
    WHERE n < 100
)
SELECT * FROM t;
ERROR:  cannot extract attribute from empty tuple slot

WITH RECURSIVE t(n) AS (
    VALUES (1)
UNION ALL
    SELECT n+1
    FROM t
    WHERE n < 100
)
SELECT * FROM t;
ERROR:  cannot extract attribute from empty tuple slot

Cheers,
David.
--
David Fetter <david@fetter.org> http://fetter.org/
Phone: +1 415 235 3778  AIM: dfetter666  Yahoo!: dfetter
Skype: davidfetter      XMPP: david.fetter@gmail.com

Remember to vote!
Consider donating to Postgres: http://www.postgresql.org/about/donate

Re: WITH RECURSIVE patch V0.1

From
Zoltan Boszormenyi
Date:
David Fetter írta:
> On Sun, May 18, 2008 at 08:51:29PM +0900, Tatsuo Ishii wrote:
>
>> WITH RECURSIVE patch V0.1
>>
>> Here are patches to implement WITH RECURSIVE clause. There are some
>> limitiations and TODO items(see the "Current limitations" section
>> below). Comments are welcome.
>>
>> 1. Credit
>>
>> These patches were developed by Yoshiyuki Asaba (y-asab@sraoss.co.jp)
>> with some discussions with Tatsuo Ishii (ishii@sraoss.co.jp).
>>
>
> This is really great!  Kudos to all who made this happen :)
>
> I tried a bunch of different queries, and so far, only these two
> haven't worked.  Any ideas what I'm doing wrong here?
>
> WITH RECURSIVE t(n) AS (
>     SELECT 1
> UNION ALL
>     SELECT n+1
>     FROM t
>     WHERE n < 100
> )
> SELECT * FROM t;
> ERROR:  cannot extract attribute from empty tuple slot
>
> WITH RECURSIVE t(n) AS (
>     VALUES (1)
> UNION ALL
>     SELECT n+1
>     FROM t
>     WHERE n < 100
> )
> SELECT * FROM t;
> ERROR:  cannot extract attribute from empty tuple slot
>
> Cheers,
> David.
>

Here's a test case attached shamelessly stolen from
http://www.adp-gmbh.ch/ora/sql/connect_by.html

This query (without naming toplevel columns) works:

# with recursive x as (select * from test_connect_by where parent is
null union all select base.* from test_connect_by as base, x where
base.parent = x.child) select * from x;
 parent | child
--------+-------
        |    38
        |    26
        |    18
     18 |    11
     18 |     7
     26 |    13
     26 |     1
     26 |    12
     38 |    15
     38 |    17
     38 |     6
     17 |     9
     17 |     8
     15 |    10
     15 |     5
      5 |     2
      5 |     3
(17 rows)

It even works when I add my "level" column:

# with recursive x(level, parent, child) as (select 1::bigint, * from
test_connect_by where parent is null union all select x.level + 1,
base.* from test_connect_by as base, x where base.parent = x.child)
select * from x;
 level | parent | child
-------+--------+-------
     1 |        |    38
     1 |        |    26
     1 |        |    18
     2 |     18 |    11
     2 |     18 |     7
     2 |     26 |    13
     2 |     26 |     1
     2 |     26 |    12
     2 |     38 |    15
     2 |     38 |    17
     2 |     38 |     6
     3 |     17 |     9
     3 |     17 |     8
     3 |     15 |    10
     3 |     15 |     5
     4 |      5 |     2
     4 |      5 |     3
(17 rows)

But I have a little problem with the output.
If it's not obvious, here is the query tweaked a little below.

# with recursive x(level, parent, child) as (select 1::integer, * from
test_connect_by where parent is null union all select x.level + 1,
base.* from test_connect_by as base, x where base.parent = x.child)
select lpad(' ', 4*level - 1) || child from x;
     ?column?
------------------
    38
    26
    18
        11
        7
        13
        1
        12
        15
        17
        6
            9
            8
            10
            5
                2
                3
(17 rows)

Can we get the rows in tree order, please? I.e. something like this:

     ?column?
------------------
    38
        15
            10
            5
                2
                3
        17
            9
            8
        6
    26
        13
        1
        12
    18
        11
        7
(17 rows)

After all, I didn't specify any ORDER BY clauses in the base, recursive
or the final queries.

Also, it seems there are no infinite recursion detection:

# with recursive x(level, parent, child) as (
    select 1::integer, * from test_connect_by where parent is null
    union all
    select x.level + 1, base.* from test_connect_by as base, x where
base.child = x.child
) select * from x;
... it waits and waits and waits ...

Also, there's another rough edge:

# with recursive x as (select * from test_connect_by where parent is
null) select * from x;
server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.
The connection to the server was lost. Attempting reset: Succeeded.

Best regards,
Zoltán Böszörményi

--
----------------------------------
Zoltán Böszörményi
Cybertec Schönig & Schönig GmbH
http://www.postgresql.at/

create table test_connect_by (
  parent     integer,
  child      integer,
  constraint uq_tcb unique (child)
);

insert into test_connect_by values ( 5, 2);
insert into test_connect_by values ( 5, 3);

insert into test_connect_by values (18,11);
insert into test_connect_by values (18, 7);

insert into test_connect_by values (17, 9);
insert into test_connect_by values (17, 8);

insert into test_connect_by values (26,13);
insert into test_connect_by values (26, 1);
insert into test_connect_by values (26,12);

insert into test_connect_by values (15,10);
insert into test_connect_by values (15, 5);

insert into test_connect_by values (38,15);
insert into test_connect_by values (38,17);
insert into test_connect_by values (38, 6);

insert into test_connect_by values (null, 38);
insert into test_connect_by values (null, 26);
insert into test_connect_by values (null, 18);

Re: WITH RECURSIVE patch V0.1

From
"Merlin Moncure"
Date:
On Sun, May 18, 2008 at 5:22 PM, Zoltan Boszormenyi <zb@cybertec.at> wrote:
> Can we get the rows in tree order, please? I.e. something like this:

Is ordering by tree order defined in the standard when no explicit
order is given?  If not, it probably returns them in the order they
are pulled up, which might be the fastest way.

merlin

Re: [HACKERS] WITH RECURSIVE patch V0.1

From
Mark Mielke
Date:
Merlin Moncure wrote: 
On Sun, May 18, 2008 at 5:22 PM, Zoltan Boszormenyi <zb@cybertec.at> wrote: 
Can we get the rows in tree order, please? I.e. something like this:   
Is ordering by tree order defined in the standard when no explicit
order is given?  If not, it probably returns them in the order they
are pulled up, which might be the fastest way

+1 for the fastest way, which I expect to often be "find all level 1 matches", "find all level 2 matches", ... If ORDER BY is important, it should be specified (although it may be difficult or impossible to properly represent ORDER BY for a tree? not sure?) I think most uses of recursive require extra client side code to deal with anyways, so only relative order is important (order within a particular branch).

There are things I'd like to use this for right now. Currently I use plpgsql procedures to implement my own recursion. :-)

Cheers,
mark

-- 
Mark Mielke <mark@mielke.cc>

Re: WITH RECURSIVE patch V0.1

From
David Fetter
Date:
On Mon, May 19, 2008 at 12:21:20AM -0400, Gregory Stark wrote:
> "Zoltan Boszormenyi" <zb@cybertec.at> writes:
> > Also, it seems there are no infinite recursion detection:
> >
> > # with recursive x(level, parent, child) as (
> >    select 1::integer, * from test_connect_by where parent is null
> >    union all
> >    select x.level + 1, base.* from test_connect_by as base, x where base.child
> > = x.child
> > ) select * from x;
> > ... it waits and waits and waits ...
>
> Well, psql might wait and wait but it's actually receiving rows.  A
> cleverer client should be able to deal with infinite streams of
> records.

That would be a very good thing for libpq (and its descendants) to
have :)

> I think DB2 does produce a warning if there is no clause it can
> determine will bound the results.  But that's not actually reliable.

I'd think not, as it's (in some sense) a Halting Problem.

> It's quite possible to have clauses which will limit the output but
> not in a way the database can determine.  Consider for example a
> tree-traversal for a binary tree stored in a recursive table
> reference.  The DBA might know that the data contains no loops but
> the database doesn't.

I seem to recall Oracle's implementation can do this traversal on
write operations, but maybe that's just their marketing.

Cheers,
David.
--
David Fetter <david@fetter.org> http://fetter.org/
Phone: +1 415 235 3778  AIM: dfetter666  Yahoo!: dfetter
Skype: davidfetter      XMPP: david.fetter@gmail.com

Remember to vote!
Consider donating to Postgres: http://www.postgresql.org/about/donate

Re: WITH RECURSIVE patch V0.1

From
Gregory Stark
Date:
"David Fetter" <david@fetter.org> writes:

> On Mon, May 19, 2008 at 12:21:20AM -0400, Gregory Stark wrote:
>> "Zoltan Boszormenyi" <zb@cybertec.at> writes:
>> > Also, it seems there are no infinite recursion detection:
>> >
>> > # with recursive x(level, parent, child) as (
>> >    select 1::integer, * from test_connect_by where parent is null
>> >    union all
>> >    select x.level + 1, base.* from test_connect_by as base, x where base.child
>> > = x.child
>> > ) select * from x;
>> > ... it waits and waits and waits ...
>>
>> Well, psql might wait and wait but it's actually receiving rows.  A
>> cleverer client should be able to deal with infinite streams of
>> records.
>
> That would be a very good thing for libpq (and its descendants) to
> have :)

I think you can do it in libpq.

In psql you can use \set FETCH_COUNT or somrthing like this (I can't look it
up just now) to use a cursor to do this too.


--
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com
  Ask me about EnterpriseDB's 24x7 Postgres support!

Re: WITH RECURSIVE patch V0.1

From
Gregory Stark
Date:
This is indeed really cool. I'm sorry I haven't gotten to doing what I
promised in this area but I'm glad it's happening anyways.


"Zoltan Boszormenyi" <zb@cybertec.at> writes:

> Can we get the rows in tree order, please?
>...
> After all, I didn't specify any ORDER BY clauses in the base, recursive or the
> final queries.

The standard has a clause to specify depth-first order. However doing a
depth-first traversal would necessitate quite a different looking plan and
it's far less obvious (to me anyways) how to do it.

> Also, it seems there are no infinite recursion detection:
>
> # with recursive x(level, parent, child) as (
>    select 1::integer, * from test_connect_by where parent is null
>    union all
>    select x.level + 1, base.* from test_connect_by as base, x where base.child
> = x.child
> ) select * from x;
> ... it waits and waits and waits ...

Well, psql might wait and wait but it's actually receiving rows. A cleverer
client should be able to deal with infinite streams of records.

I think DB2 does produce a warning if there is no clause it can determine will
bound the results. But that's not actually reliable. It's quite possible to
have clauses which will limit the output but not in a way the database can
determine. Consider for example a tree-traversal for a binary tree stored in a
recursive table reference. The DBA might know that the data contains no loops
but the database doesn't.

--
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com
  Ask me about EnterpriseDB's PostGIS support!

Re: WITH RECURSIVE patch V0.1

From
Zoltan Boszormenyi
Date:
Gregory Stark írta:
> This is indeed really cool. I'm sorry I haven't gotten to doing what I
> promised in this area but I'm glad it's happening anyways.
>
>
> "Zoltan Boszormenyi" <zb@cybertec.at> writes:
>
>
>> Can we get the rows in tree order, please?
>> ...
>> After all, I didn't specify any ORDER BY clauses in the base, recursive or the
>> final queries.
>>
>
> The standard has a clause to specify depth-first order. However doing a
> depth-first traversal would necessitate quite a different looking plan and
> it's far less obvious (to me anyways) how to do it.
>

That would be even cooler to have it implemented as well.

>> Also, it seems there are no infinite recursion detection:
>>
>> # with recursive x(level, parent, child) as (
>>    select 1::integer, * from test_connect_by where parent is null
>>    union all
>>    select x.level + 1, base.* from test_connect_by as base, x where base.child
>> = x.child
>> ) select * from x;
>> ... it waits and waits and waits ...
>>
>
> Well, psql might wait and wait but it's actually receiving rows. A cleverer
> client should be able to deal with infinite streams of records.
>

I think it's the other way around. The server should not emit infinite
number of records.

> I think DB2 does produce a warning if there is no clause it can determine will
> bound the results. But that's not actually reliable. It's quite possible to
> have clauses which will limit the output but not in a way the database can
> determine. Consider for example a tree-traversal for a binary tree stored in a
> recursive table reference. The DBA might know that the data contains no loops
> but the database doesn't.
>

Well, a maintenance resjunk could be used like the branch column in
tablefunc::connectby().

--
----------------------------------
Zoltán Böszörményi
Cybertec Schönig & Schönig GmbH
http://www.postgresql.at/

Re: [HACKERS] WITH RECURSIVE patch V0.1

From
Martijn van Oosterhout
Date:
On Mon, May 19, 2008 at 08:19:17AM +0200, Zoltan Boszormenyi wrote:
> >The standard has a clause to specify depth-first order. However doing a
> >depth-first traversal would necessitate quite a different looking plan and
> >it's far less obvious (to me anyways) how to do it.
>
> That would be even cooler to have it implemented as well.

From an implementation point of view, the only difference between
breadth-first and depth-first is that your tuplestore needs to be LIFO
instead of FIFO. However, just looking at the plan I don't know whether
it could support that kind of usage. At the very least I don't think
the standard tuplestore code can handle it.

> >Well, psql might wait and wait but it's actually receiving rows. A cleverer
> >client should be able to deal with infinite streams of records.
>
> I think it's the other way around. The server should not emit infinite
> number of records.

The server won't, the universe will end first. This is a nice example
of the halting problem:

http://en.wikipedia.org/wiki/Halting_problem

Which was proved unsolvable a long time ago.

Have a nice day,
--
Martijn van Oosterhout   <kleptog@svana.org>   http://svana.org/kleptog/
> Please line up in a tree and maintain the heap invariant while
> boarding. Thank you for flying nlogn airlines.
Attachment

Re: WITH RECURSIVE patch V0.1

From
Yoshiyuki Asaba
Date:
Hi,

From: Zoltan Boszormenyi <zb@cybertec.at>
Subject: Re: [PATCHES] WITH RECURSIVE patch V0.1
Date: Mon, 19 May 2008 08:19:17 +0200

> >> Also, it seems there are no infinite recursion detection:
> >>
> >> # with recursive x(level, parent, child) as (
> >>    select 1::integer, * from test_connect_by where parent is null
> >>    union all
> >>    select x.level + 1, base.* from test_connect_by as base, x where base.child
> >> = x.child
> >> ) select * from x;
> >> ... it waits and waits and waits ...
> >>
> >
> > Well, psql might wait and wait but it's actually receiving rows. A cleverer
> > client should be able to deal with infinite streams of records.
> >
>
> I think it's the other way around. The server should not emit infinite
> number of records.

How about adding new GUC parameter "max_recursive_call"?

Regards,
--
Yoshiyuki Asaba
y-asaba@sraoss.co.jp

Re: WITH RECURSIVE patch V0.1

From
Zoltan Boszormenyi
Date:
Yoshiyuki Asaba írta:
> Hi,
>
> From: Zoltan Boszormenyi <zb@cybertec.at>
> Subject: Re: [PATCHES] WITH RECURSIVE patch V0.1
> Date: Mon, 19 May 2008 08:19:17 +0200
>
>
>>>> Also, it seems there are no infinite recursion detection:
>>>>
>>>> # with recursive x(level, parent, child) as (
>>>>    select 1::integer, * from test_connect_by where parent is null
>>>>    union all
>>>>    select x.level + 1, base.* from test_connect_by as base, x where base.child
>>>> = x.child
>>>> ) select * from x;
>>>> ... it waits and waits and waits ...
>>>>
>>>>
>>> Well, psql might wait and wait but it's actually receiving rows. A cleverer
>>> client should be able to deal with infinite streams of records.
>>>
>>>
>> I think it's the other way around. The server should not emit infinite
>> number of records.
>>
>
> How about adding new GUC parameter "max_recursive_call"?
>

Yes, why not?
MSSQL has a similar MAXRECURSION hint for WITH RECURSIVE queries
according to their docs.
http://msdn.microsoft.com/en-us/library/ms186243.aspx

> Regards,
> --
> Yoshiyuki Asaba
> y-asaba@sraoss.co.jp
>
>


--
----------------------------------
Zoltán Böszörményi
Cybertec Schönig & Schönig GmbH
http://www.postgresql.at/

Re: [HACKERS] WITH RECURSIVE patch V0.1

From
Zoltan Boszormenyi
Date:
Martijn van Oosterhout írta:
> On Mon, May 19, 2008 at 08:19:17AM +0200, Zoltan Boszormenyi wrote:
>
>>> The standard has a clause to specify depth-first order. However doing a
>>> depth-first traversal would necessitate quite a different looking plan and
>>> it's far less obvious (to me anyways) how to do it.
>>>
>> That would be even cooler to have it implemented as well.
>>
>
> From an implementation point of view, the only difference between
> breadth-first and depth-first is that your tuplestore needs to be LIFO
> instead of FIFO. However, just looking at the plan I don't know whether
> it could support that kind of usage. At the very least I don't think
> the standard tuplestore code can handle it.
>
>
>>> Well, psql might wait and wait but it's actually receiving rows. A cleverer
>>> client should be able to deal with infinite streams of records.
>>>
>> I think it's the other way around. The server should not emit infinite
>> number of records.
>>
>
> The server won't, the universe will end first.

The universe is alive and well, thank you. :-)
But the server won't emit infinite number of records, you are right.
Given the implementation uses a tuplestore and not producing the
tupleslots on the fly, it will go OOM first not the psql client,
I watched them in 'top'. It just takes a bit of time.

>  This is a nice example
> of the halting problem:
>
> http://en.wikipedia.org/wiki/Halting_problem
>
> Which was proved unsolvable a long time ago.
>

Hmpf, yes, I forgot too much about Turing-machines since university. :-(

> Have a nice day,
>

--
----------------------------------
Zoltán Böszörményi
Cybertec Schönig & Schönig GmbH
http://www.postgresql.at/

Re: [HACKERS] WITH RECURSIVE patch V0.1

From
Zoltan Boszormenyi
Date:
Martijn van Oosterhout írta:
> On Mon, May 19, 2008 at 08:19:17AM +0200, Zoltan Boszormenyi wrote:
>
>>> The standard has a clause to specify depth-first order. However doing a
>>> depth-first traversal would necessitate quite a different looking plan and
>>> it's far less obvious (to me anyways) how to do it.
>>>
>> That would be even cooler to have it implemented as well.
>>
>
> From an implementation point of view, the only difference between
> breadth-first and depth-first is that your tuplestore needs to be LIFO
> instead of FIFO.

Are you sure? I think a LIFO tuplestore would simply return reversed
breadth-first order. Depth-first means for every new record descend into
another recursion first then continue with the next record on the right.

> However, just looking at the plan I don't know whether
> it could support that kind of usage. At the very least I don't think
> the standard tuplestore code can handle it.
>


--
----------------------------------
Zoltán Böszörményi
Cybertec Schönig & Schönig GmbH
http://www.postgresql.at/

Re: [HACKERS] WITH RECURSIVE patch V0.1

From
Martijn van Oosterhout
Date:
On Mon, May 19, 2008 at 11:56:17AM +0200, Zoltan Boszormenyi wrote:
> >From an implementation point of view, the only difference between
> >breadth-first and depth-first is that your tuplestore needs to be LIFO
> >instead of FIFO.
>
> Are you sure? I think a LIFO tuplestore would simply return reversed
> breadth-first order. Depth-first means for every new record descend into
> another recursion first then continue with the next record on the right.

Say your tree looks like:
Root->A, D
A->B,C
D->E,F

LIFO pushes A and D. It then pops A and pushes B and C. B and C have no
children and are returned. Then D is popped and E and F pushed. So the
returned order is: A,B,C,D,E,F. You could also do B,C,A,E,F,D if you
wanted.

FIFO pushes A and D. It then pops A and puts B and C at *the end*. It
then pops D and pushes E and F at the end. So you get the order
A,D,B,C,E,F

Hope this helps,
--
Martijn van Oosterhout   <kleptog@svana.org>   http://svana.org/kleptog/
> Please line up in a tree and maintain the heap invariant while
> boarding. Thank you for flying nlogn airlines.

Re: [HACKERS] WITH RECURSIVE patch V0.1

From
Zoltan Boszormenyi
Date:
Martijn van Oosterhout írta:
> On Mon, May 19, 2008 at 11:56:17AM +0200, Zoltan Boszormenyi wrote:
>
>> >From an implementation point of view, the only difference between
>>
>>> breadth-first and depth-first is that your tuplestore needs to be LIFO
>>> instead of FIFO.
>>>
>> Are you sure? I think a LIFO tuplestore would simply return reversed
>> breadth-first order. Depth-first means for every new record descend into
>> another recursion first then continue with the next record on the right.
>>
>
> Say your tree looks like:
> Root->A, D
> A->B,C
> D->E,F
>
> LIFO pushes A and D. It then pops A and pushes B and C. B and C have no
> children and are returned. Then D is popped and E and F pushed. So the
> returned order is: A,B,C,D,E,F. You could also do B,C,A,E,F,D if you
> wanted.
>
> FIFO pushes A and D. It then pops A and puts B and C at *the end*. It
> then pops D and pushes E and F at the end. So you get the order
> A,D,B,C,E,F
>
> Hope this helps,
>

Thanks, I didn't consider popping elements off while processing.
However, if the toplevel query returns tuples in A, D order, you need
a positioned insert into the tuplestore, because the LIFO would pop D first.

Say, a "treestore" would work this way:
1. setup: treestore is empty, storage_position := 0
2. treestore_puttupleslot() adds slot at current position,
storage_position++
3. treestore_gettupleslot() removes slot from the beginning,
storage_position := 0
This works easily in memory lists but it's not obvious for me how it may
work
with disk backed temporary storage inside PG.

--
----------------------------------
Zoltán Böszörményi
Cybertec Schönig & Schönig GmbH
http://www.postgresql.at/

Re: WITH RECURSIVE patch V0.1

From
David Fetter
Date:
On Mon, May 19, 2008 at 05:57:17PM +0900, Yoshiyuki Asaba wrote:
> Hi,
>
> > I think it's the other way around. The server should not emit
> > infinite number of records.
>
> How about adding new GUC parameter "max_recursive_call"?

Couldn't we just have it pay attention to the existing
max_stack_depth?

Cheers,
David.
--
David Fetter <david@fetter.org> http://fetter.org/
Phone: +1 415 235 3778  AIM: dfetter666  Yahoo!: dfetter
Skype: davidfetter      XMPP: david.fetter@gmail.com

Remember to vote!
Consider donating to Postgres: http://www.postgresql.org/about/donate

Re: [HACKERS] WITH RECURSIVE patch V0.1

From
Hannu Krosing
Date:
On Sun, 2008-05-18 at 22:17 -0700, David Fetter wrote:
> On Mon, May 19, 2008 at 12:21:20AM -0400, Gregory Stark wrote:
> > "Zoltan Boszormenyi" <zb@cybertec.at> writes:
> > > Also, it seems there are no infinite recursion detection:
> > >
> > > # with recursive x(level, parent, child) as (
> > >    select 1::integer, * from test_connect_by where parent is null
> > >    union all
> > >    select x.level + 1, base.* from test_connect_by as base, x where base.child
> > > = x.child
> > > ) select * from x;
> > > ... it waits and waits and waits ...
> >
> > Well, psql might wait and wait but it's actually receiving rows.  A
> > cleverer client should be able to deal with infinite streams of
> > records.
>
> That would be a very good thing for libpq (and its descendants) to
> have :)
>
> > I think DB2 does produce a warning if there is no clause it can
> > determine will bound the results.  But that's not actually reliable.
>
> I'd think not, as it's (in some sense) a Halting Problem.
>
> > It's quite possible to have clauses which will limit the output but
> > not in a way the database can determine.  Consider for example a
> > tree-traversal for a binary tree stored in a recursive table
> > reference.  The DBA might know that the data contains no loops but
> > the database doesn't.
>
> I seem to recall Oracle's implementation can do this traversal on
> write operations, but maybe that's just their marketing.

It may be possible to solve at least some of it by doing something
similar to hash version of DISTINCT by having an hashtable of tuples
already returned and not descending branches where you have already
been.

> Cheers,
> David.
> --
> David Fetter <david@fetter.org> http://fetter.org/
> Phone: +1 415 235 3778  AIM: dfetter666  Yahoo!: dfetter
> Skype: davidfetter      XMPP: david.fetter@gmail.com
>
> Remember to vote!
> Consider donating to Postgres: http://www.postgresql.org/about/donate
>

Re: [HACKERS] WITH RECURSIVE patch V0.1

From
Gregory Stark
Date:
"Martijn van Oosterhout" <kleptog@svana.org> writes:

> From an implementation point of view, the only difference between
> breadth-first and depth-first is that your tuplestore needs to be LIFO
> instead of FIFO.

I think it's not so simple. How do you reconcile that concept with the join
plans like merge join or hash join which expect you to be able to be able to
process the records in a specific order?

It sounds like you might have to keep around a stack of started executor nodes
or something but hopefully we can avoid anything like that because, well, ick.

--
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com
  Ask me about EnterpriseDB's PostGIS support!

Re: [HACKERS] WITH RECURSIVE patch V0.1

From
Zoltan Boszormenyi
Date:
Gregory Stark írta:
> "Martijn van Oosterhout" <kleptog@svana.org> writes:
>
>
>> From an implementation point of view, the only difference between
>> breadth-first and depth-first is that your tuplestore needs to be LIFO
>> instead of FIFO.
>>
>
> I think it's not so simple. How do you reconcile that concept with the join
> plans like merge join or hash join which expect you to be able to be able to
> process the records in a specific order?
>
> It sounds like you might have to keep around a stack of started executor nodes
> or something but hopefully we can avoid anything like that because, well, ick.
>

If I understand the code right, the recursion from level N to level N+1
goes like this:
collect all records from level N and JOIN it with the recursive query.
This way
we get all level 1 records from the base query, then all records at the
second level, etc.
This is how it gets breadth-first ordering.
Depth-first ordering could go like this: get only 1 from the current
level then go
into recursion. Repeat until there are no records in the current level.
The only difference would be more recursion steps. Instead of one per level,
there would be N per level if there are N tuples in the current level.
Definitely
slower then the current implementation but comparable with the tablefunc.c
connectby() code.

--
----------------------------------
Zoltán Böszörményi
Cybertec Schönig & Schönig GmbH
http://www.postgresql.at/

Re: WITH RECURSIVE patch V0.1

From
Kev
Date:
On May 19, 1:17 am, da...@fetter.org (David Fetter) wrote:
> On Mon, May 19, 2008 at 12:21:20AM -0400, Gregory Stark wrote:
> > It's quite possible to have clauses which will limit the output but
> > not in a way the database can determine.  Consider for example a
> > tree-traversal for a binary tree stored in a recursive table
> > reference.  The DBA might know that the data contains no loops but
> > the database doesn't.
>
> I seem to recall Oracle's implementation can do this traversal on
> write operations, but maybe that's just their marketing.

That's how I implement (id, name, parent)-trees as a DBA, having an
insert/update trigger function check_no_loops(), but I'm not sure that
it would be faster than the hash method suggested by Hannu Krosing.  I
guess it depends on whether you're inserting/updating or selecting
more.  Does it make sense to leave the option to the user, whether to
check for infinite recursion just in time or not?

Kev

Re: WITH RECURSIVE patch V0.1

From
Tatsuo Ishii
Date:
> On Sun, May 18, 2008 at 08:51:29PM +0900, Tatsuo Ishii wrote:
> > WITH RECURSIVE patch V0.1
> >
> > Here are patches to implement WITH RECURSIVE clause. There are some
> > limitiations and TODO items(see the "Current limitations" section
> > below). Comments are welcome.
> >
> > 1. Credit
> >
> > These patches were developed by Yoshiyuki Asaba (y-asab@sraoss.co.jp)
> > with some discussions with Tatsuo Ishii (ishii@sraoss.co.jp).
>
> This is really great!  Kudos to all who made this happen :)

Thanks. In addition to above, Sumitomo Electric Information Systems
Co., and SRA OSS, Inc. Japan made this happen.

I and Yoshiyuki Asaba are now in Ottawa to join PGCon. I hope to have
some discussions on this here with anyone who are interested in this.
--
Tatsuo Ishii
SRA OSS, Inc. Japan

> I tried a bunch of different queries, and so far, only these two
> haven't worked.  Any ideas what I'm doing wrong here?
>
> WITH RECURSIVE t(n) AS (
>     SELECT 1
> UNION ALL
>     SELECT n+1
>     FROM t
>     WHERE n < 100
> )
> SELECT * FROM t;
> ERROR:  cannot extract attribute from empty tuple slot
>
> WITH RECURSIVE t(n) AS (
>     VALUES (1)
> UNION ALL
>     SELECT n+1
>     FROM t
>     WHERE n < 100
> )
> SELECT * FROM t;
> ERROR:  cannot extract attribute from empty tuple slot
>
> Cheers,
> David.
> --
> David Fetter <david@fetter.org> http://fetter.org/
> Phone: +1 415 235 3778  AIM: dfetter666  Yahoo!: dfetter
> Skype: davidfetter      XMPP: david.fetter@gmail.com
>
> Remember to vote!
> Consider donating to Postgres: http://www.postgresql.org/about/donate

Re: [HACKERS] WITH RECURSIVE patch V0.1

From
Yoshiyuki Asaba
Date:
Hi,

From: David Fetter <david@fetter.org>
Subject: Re: [HACKERS] [PATCHES] WITH RECURSIVE patch V0.1
Date: Mon, 19 May 2008 04:36:30 -0700

> > > I think it's the other way around. The server should not emit
> > > infinite number of records.
> >
> > How about adding new GUC parameter "max_recursive_call"?
>
> Couldn't we just have it pay attention to the existing
> max_stack_depth?

Recursive query does not consume stack. The server enters an infinite
loop without consuming stack. Stack-depth error does not happen.

Regards,
--
Yoshiyuki Asaba
y-asaba@sraoss.co.jp

Re: [HACKERS] WITH RECURSIVE patch V0.1

From
Gregory Stark
Date:
"Yoshiyuki Asaba" <y-asaba@sraoss.co.jp> writes:

> Hi,
>
> From: David Fetter <david@fetter.org>
> Subject: Re: [HACKERS] [PATCHES] WITH RECURSIVE patch V0.1
> Date: Mon, 19 May 2008 04:36:30 -0700
>
>> > > I think it's the other way around. The server should not emit
>> > > infinite number of records.
>> >
>> > How about adding new GUC parameter "max_recursive_call"?
>>
>> Couldn't we just have it pay attention to the existing
>> max_stack_depth?
>
> Recursive query does not consume stack. The server enters an infinite
> loop without consuming stack. Stack-depth error does not happen.

We could have a separate guc variable which limits the maximum number of
levels of recursive iterations. That might be a useful feature for DBAs that
want to limit their users from issuing an infinite query.

Note that users can always construct their query to limit the number of
recursive iterations. So this would only be useful for DBAs that don't trust
their users and want to impose a limit. It doesn't add any actual expressive
power that SQL doesn't have already.

The recursive query syntax in the spec actually does include the ability to
assign an output column to show what level of recursive iteration you're on.
So alternately we could have a GUC variable which just allows the DBA to
prohibit any recursive query without such a column and a fiter imposing a
maximum value on it. That's probably the most appropriate option.


--
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com
  Ask me about EnterpriseDB's Slony Replication support!

Re: [HACKERS] WITH RECURSIVE patch V0.1

From
"Joshua D. Drake"
Date:
> >> Couldn't we just have it pay attention to the existing
> >> max_stack_depth?
> >
> > Recursive query does not consume stack. The server enters an infinite
> > loop without consuming stack. Stack-depth error does not happen.
>
> We could have a separate guc variable which limits the maximum number of
> levels of recursive iterations. That might be a useful feature for DBAs that
> want to limit their users from issuing an infinite query.
>

statement_timeout :)

Joshua D. Drake

Re: [HACKERS] WITH RECURSIVE patch V0.1

From
Gregory Stark
Date:
"Joshua D. Drake" <jd@commandprompt.com> writes:

>> >> Couldn't we just have it pay attention to the existing
>> >> max_stack_depth?
>> >
>> > Recursive query does not consume stack. The server enters an infinite
>> > loop without consuming stack. Stack-depth error does not happen.
>>
>> We could have a separate guc variable which limits the maximum number of
>> levels of recursive iterations. That might be a useful feature for DBAs that
>> want to limit their users from issuing an infinite query.
>
> statement_timeout :)

Good point.

Though it occurs to me that if you set FETCH_COUNT in psql (or do the
equivalent in your code ) statement_timeout becomes much less useful.

--
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com
  Ask me about EnterpriseDB's On-Demand Production Tuning

Re: WITH RECURSIVE patch V0.1

From
Yoshiyuki Asaba
Date:
Hi,

From: David Fetter <david@fetter.org>
Subject: Re: [PATCHES] WITH RECURSIVE patch V0.1
Date: Sun, 18 May 2008 11:47:37 -0700

> I tried a bunch of different queries, and so far, only these two
> haven't worked.  Any ideas what I'm doing wrong here?
>
> WITH RECURSIVE t(n) AS (
>     SELECT 1
> UNION ALL
>     SELECT n+1
>     FROM t
>     WHERE n < 100
> )
> SELECT * FROM t;
> ERROR:  cannot extract attribute from empty tuple slot

Thank you for the report. I've fixed.

postgres=# WITH RECURSIVE t(n) AS (
    SELECT 1
UNION ALL
    SELECT n+1
    FROM t
    WHERE n < 100
)
SELECT count(*) FROM t;
 count
-------
   100
(1 row)

Regards,
--
Yoshiyuki Asaba
y-asaba@sraoss.co.jp

Re: WITH RECURSIVE patch V0.1

From
David Fetter
Date:
On Sat, May 24, 2008 at 03:21:01AM +0900, Yoshiyuki Asaba wrote:
> Hi,
>
> From: David Fetter <david@fetter.org>
> Subject: Re: [PATCHES] WITH RECURSIVE patch V0.1
> Date: Sun, 18 May 2008 11:47:37 -0700
>
> > I tried a bunch of different queries, and so far, only these two
> > haven't worked.  Any ideas what I'm doing wrong here?
> >
> > WITH RECURSIVE t(n) AS (
> >     SELECT 1
> > UNION ALL
> >     SELECT n+1
> >     FROM t
> >     WHERE n < 100
> > )
> > SELECT * FROM t;
> > ERROR:  cannot extract attribute from empty tuple slot
>
> Thank you for the report. I've fixed.
>
> postgres=# WITH RECURSIVE t(n) AS (
>     SELECT 1
> UNION ALL
>     SELECT n+1
>     FROM t
>     WHERE n < 100
> )
> SELECT count(*) FROM t;
>  count
> -------
>    100
> (1 row)
>
> Regards,
> --
> Yoshiyuki Asaba
> y-asaba@sraoss.co.jp

Great!

Where is the new patch?

Cheers,
David.
--
David Fetter <david@fetter.org> http://fetter.org/
Phone: +1 415 235 3778  AIM: dfetter666  Yahoo!: dfetter
Skype: davidfetter      XMPP: david.fetter@gmail.com

Remember to vote!
Consider donating to Postgres: http://www.postgresql.org/about/donate

Re: WITH RECURSIVE patch V0.1

From
Yoshiyuki Asaba
Date:
Hi,

From: David Fetter <david@fetter.org>
Subject: Re: [PATCHES] WITH RECURSIVE patch V0.1
Date: Fri, 23 May 2008 11:26:30 -0700

> Where is the new patch?

I will create the revised patch on June.
This is a patch for this problem.

*** ../../pgsql/src/backend/executor/nodeRecursivescan.c    2008-05-24 04:45:23.000000000 +0900
--- src/backend/executor/nodeRecursivescan.c    2008-05-24 04:47:54.000000000 +0900
***************
*** 37,43 ****
          node->ss.ps.state->es_tuplestorestate = tuplestore_begin_heap(true, false, work_mem);
      }

!     slot = node->ss.ps.ps_ResultTupleSlot;
      if (tuplestore_gettupleslot(node->ss.ps.state->es_tuplestorestate, true, slot))
          return slot;

--- 37,43 ----
          node->ss.ps.state->es_tuplestorestate = tuplestore_begin_heap(true, false, work_mem);
      }

!     slot = node->ss.ss_ScanTupleSlot;
      if (tuplestore_gettupleslot(node->ss.ps.state->es_tuplestorestate, true, slot))
          return slot;


Regards,
--
Yoshiyuki Asaba
y-asaba@sraoss.co.jp

Re: [HACKERS] WITH RECURSIVE patch V0.1

From
David Fetter
Date:
On Sat, May 24, 2008 at 05:01:11AM +0900, Yoshiyuki Asaba wrote:
> Hi,
>
> From: David Fetter <david@fetter.org>
> Subject: Re: [PATCHES] WITH RECURSIVE patch V0.1
> Date: Fri, 23 May 2008 11:26:30 -0700
>
> > Where is the new patch?
>
> I will create the revised patch on June.  This is a patch for this
> problem.

Thanks very much :)

Cheers,
David.
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Re: WITH RECURSIVE patch V0.1

From
Yoshiyuki Asaba
Date:
Hi,

From: Zoltan Boszormenyi <zb@cybertec.at>
Subject: Re: [PATCHES] WITH RECURSIVE patch V0.1
Date: Sun, 18 May 2008 23:22:02 +0200

> But I have a little problem with the output.
> If it's not obvious, here is the query tweaked a little below.
...
> Can we get the rows in tree order, please? I.e. something like this:
>
>      ?column?
> ------------------
>     38
>         15
>             10
>             5
>                 2
>                 3
>         17
>             9
>             8
>         6
>     26
>         13
>         1
>         12
>     18
>         11
>         7
> (17 rows)

No, you can't. However, you can obtain recursive path by using ARRAY
type, as another way. Here is a sample SQL.

WITH RECURSIVE x(level, parent, child, path) AS
  (SELECT 1::integer, * , array[child] FROM test_connect_by
      WHERE parent IS NULL

   UNION ALL

   SELECT x.level + 1, base.*, array_append(path, base.child)
     FROM test_connect_by AS base, x WHERE base.parent = x.child
  )
SELECT path, array_to_string(path, '->') FROM x
  WHERE NOT EXISTS (SELECT 1 FROM test_connect_by WHERE parent =
     x.child);

    path     | array_to_string
-------------+-----------------
 {18,11}     | 18->11
 {18,7}      | 18->7
 {26,13}     | 26->13
 {26,1}      | 26->1
 {26,12}     | 26->12
 {38,6}      | 38->6
 {38,17,9}   | 38->17->9
 {38,17,8}   | 38->17->8
 {38,15,10}  | 38->15->10
 {38,15,5,2} | 38->15->5->2
 {38,15,5,3} | 38->15->5->3
(11 rows)

Regards,
--
Yoshiyuki Asaba
y-asaba@sraoss.co.jp

Updated patch (Re: WITH RECURSIVE patch V0.1)

From
David Fetter
Date:
On Sun, May 18, 2008 at 08:51:29PM +0900, Tatsuo Ishii wrote:
> WITH RECURSIVE patch V0.1

Please find updated patch with bug fixes from Yoshiyuki Asaba and
Michael Meskes.  Any mistakes in it are mine. :)

Cheers,
David.
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Re: [HACKERS] WITH RECURSIVE patch V0.1

From
Hans-Juergen Schoenig
Date:
Gregory Stark wrote:
> "Joshua D. Drake" <jd@commandprompt.com> writes:
>
>
>>>>> Couldn't we just have it pay attention to the existing
>>>>> max_stack_depth?
>>>>>
>>>> Recursive query does not consume stack. The server enters an infinite
>>>> loop without consuming stack. Stack-depth error does not happen.
>>>>
>>> We could have a separate guc variable which limits the maximum number of
>>> levels of recursive iterations. That might be a useful feature for DBAs that
>>> want to limit their users from issuing an infinite query.
>>>
>> statement_timeout :)
>>
>
> Good point.
>
> Though it occurs to me that if you set FETCH_COUNT in psql (or do the
> equivalent in your code ) statement_timeout becomes much less useful.
>
>

i don't think statement_timeout is a good idea at all.
it is not deterministic. depending on the load on the server some
queries will execute while others fail.
a separate GUC is needed.

    best regards,

       hans



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Re: [HACKERS] WITH RECURSIVE patch V0.1

From
"Joshua D. Drake"
Date:
Hans-Juergen Schoenig wrote:
> Gregory Stark wrote:
>> "Joshua D. Drake" <jd@commandprompt.com> writes:

>>
>
> i don't think statement_timeout is a good idea at all.
> it is not deterministic. depending on the load on the server some
> queries will execute while others fail.
> a separate GUC is needed.

I don't think we need to add clutter to GUC for something that exists to
handle the problem at hand. If our real concern is server utilization
based on user or query resources we need to look at an overall solution
for that issue not a one off for a single feature.

Sincerely,

Joshua D. Drake

Re: [HACKERS] WITH RECURSIVE patch V0.1

From
Tom Lane
Date:
[ catching up on back email ]

Gregory Stark <stark@enterprisedb.com> writes:
> "Yoshiyuki Asaba" <y-asaba@sraoss.co.jp> writes:
>> Recursive query does not consume stack. The server enters an infinite
>> loop without consuming stack. Stack-depth error does not happen.

> We could have a separate guc variable which limits the maximum number of
> levels of recursive iterations. That might be a useful feature for DBAs that
> want to limit their users from issuing an infinite query.

This whole thread seems to be proposing more and more complicated
solutions for what is really a non-problem given Yoshiyuki-san's point.
It's trivial to construct SQL queries that will run for longer than the
MTBF of your hardware --- eg, forget a few join constraints.  We've
gotten along fine with nothing but query cancel and statement_timeout
for that, and I've seen no one proposing that we need to "fix it".
We don't disallow you from writing an infinite loop in plpgsql, either.

I think this patch is plenty complicated enough without adding useless
restrictive options.

            regards, tom lane

Re: Updated patch (Re: WITH RECURSIVE patch V0.1)

From
David Fetter
Date:
On Sat, May 24, 2008 at 05:08:51AM -0700, David Fetter wrote:
> On Sun, May 18, 2008 at 08:51:29PM +0900, Tatsuo Ishii wrote:
> > WITH RECURSIVE patch V0.1
>
> Please find updated patch with bug fixes from Yoshiyuki Asaba and
> Michael Meskes.  Any mistakes in it are mine. :)

As promised, the mistakes were mine, and I did not include Asaba-san's
fix. :P

Please find enclosed another patch remedying this problem and
documenting some of what WITH [RECURSIVE] does.

I'm also thinking there should be updates to psql's \h and tab
completion code.

Cheers,
David.
--
David Fetter <david@fetter.org> http://fetter.org/
Phone: +1 415 235 3778  AIM: dfetter666  Yahoo!: dfetter
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Attachment

Re: [HACKERS] WITH RECURSIVE patch V0.1

From
Josh Berkus
Date:
Tom,

> I think this patch is plenty complicated enough without adding useless
> restrictive options.

+1 for no additonal GUC options.

--Josh Berkus