Thread: Re: [PATCHES] WITH RECURSIVE patch V0.1
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
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);
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
Merlin Moncure wrote:
+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
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>
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
"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!
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!
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/
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.
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
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/
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/
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/
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.
Attachment
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/
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
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 >
"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!
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/
> 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
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
"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!
> >> 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
"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
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
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
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
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. -- 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
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
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
--
Cybertec Schönig & Schönig GmbH
PostgreSQL Solutions and Support
Gröhrmühlgasse 26, A-2700 Wiener Neustadt
Tel: +43/1/205 10 35 / 340
www.postgresql-support.de, www.postgresql-support.com
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
[ 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
Tom, > I think this patch is plenty complicated enough without adding useless > restrictive options. +1 for no additonal GUC options. --Josh Berkus