Thread: Re: BUG #1528: Rows returned that should be excluded by WHERE clause
I wrote:
> I think the problem can be expressed as
> regression=# select 2 as id, max(b) from t2 having 2 = 1;
> id | max
> ----+-----
> 2 |
> (1 row)
> the issue is clearly that the known-false HAVING clause is pushed down
> inside the aggregation, as though it were WHERE. The existing code
> pushes down HAVING to WHERE if the clause contains no aggregates, but
> evidently this is too simplistic. What are the correct conditions for
> pushing down HAVING clauses to WHERE?
After reading the spec a little, I think that we have oversimplified our
handling of aggregate-free HAVING clauses. If you look in planner.c
you'll find that such a clause is converted into a WHERE clause, but
this is not what the spec says to do, and you can tell the difference
in cases like the above.
What the spec actually says, or at least implies, is that a HAVING
clause is to be evaluated only once per group --- where the "group"
is the whole table if there's no GROUP BY clause. The group is
to be discarded if the HAVING clause doesn't return true. SQL92 7.8:
1) Let T be the result of the preceding <from clause>, <where clause>, or <group by clause>. If that
clauseis not a <group by clause>, then T consists of a single group and does not have a grouping
column.
2) The <search condition> is applied to each group of T. The result of the <having clause> is a
groupedtable of those groups of T for which the result of the <search condition> is true.
So it's clear that what the above case should return is a grouped table
having no groups ... ie, no rows out. What we are actually returning is
one group containing no rows, which is visibly different because of the
presence of the aggregate function in the SELECT list.
There are really four cases to think about, depending on whether the
query has GROUP BY and on whether it has any aggregates outside the
HAVING clause:
1. No GROUP BY, no aggregates
Per spec, the HAVING clause should be evaluated once and either we
return the whole input or none of it. Since there are no grouped
columns and (by assumption) no aggregates in the HAVING clause, the
HAVING clause must in fact be variable-free, ie, it's a pseudoconstant
clause. (Only pseudoconstant, because it might contain outer-level
variables or volatile functions.) I think the correct implementation
in this case is to generate a gating Result node with the HAVING clause
as a one-time filter, so that we don't evaluate any of the query if the
HAVING is false. The current code gets this almost right: it will make
a variable-free WHERE clause into a Result gating condition *if it
contains no volatile functions*. So it's wrong for the volatile
function case but right otherwise.
2. GROUP BY, no aggregates
In this case the HAVING clause might contain references to the grouping
columns. It is legitimate to push down the HAVING to become WHERE,
but *only* if it doesn't contain any volatile functions --- otherwise it
might be possible to tell that the HAVING clause was executed more than
once. It would be useful to push down the HAVING if, for example, it
could become an indexscan qualifier. However if the HAVING condition
is expensive to compute (eg it contains a subselect) we'd probably be
better off not to push it into WHERE, but to arrange to evaluate it
only once per group. Right now the executor cannot support testing
such a condition, but I think it would be easy enough to improve nodeGroup.c
to allow testing a qual condition for each group.
3. No GROUP BY, has aggregates
As in case 1, the HAVING clause must be variable-free, so the best
implementation would be to put it into a gating Result node. It would
be correct to treat it the same way as we do for a HAVING clause
containing aggregates (ie, attach it as a qual condition to the Agg plan
node) --- but that would mean computing and throwing away the aggregate
result when the HAVING fails, when we could skip computing it altogether.
4. GROUP BY and has aggregates
This is really the same as case 2: we could push down the HAVING
condition if it contains no volatile functions, but unless it is
cheap to evaluate we are probably best off to attach it as a qual
condition to the Agg node, ie, evaluate it only once per group.
The only difference is that we don't need an executor fix to support
this, since Agg does quals already.
So, aside from the originally reported bug, there are two other problems
in this logic: it isn't ensuring that volatile functions will be
evaluated only once per group, and it isn't considering evaluation
cost in deciding whether a clause that could be converted to WHERE
should be or not.
I haven't yet tried to make a patch that fixes all of these things.
It'll likely come out complex enough that we don't want to back-patch
it into 8.0 or before. If so, I'll try to make a simpler variant that
fixes the semantic bugs but doesn't try to be smart about evaluation
cost.
Comments?
regards, tom lane
I wrote in reference to bug#1528:
> What the spec actually says, or at least implies, is that a HAVING
> clause is to be evaluated only once per group --- where the "group"
> is the whole table if there's no GROUP BY clause.
In fact, reading the spec more closely, it is clear that the presence
of HAVING turns the query into a grouped query even if there is no
GROUP BY. I quote SQL92 7.8 again:
7.8 <having clause>
Function
Specify a grouped table derived by the elimination of groups from ^^^^^^^^^^^^^^^^^^^^^^^ the
resultof the previously specified clause that do not meet the <search condition>.
...
1) Let T be the result of the preceding <from clause>, <where clause>, or <group by clause>. If that
clauseis not a <group by clause>, then T consists of a single group and does not have a grouping
column.
2) The <search condition> is applied to each group of T. The result of the <having clause> is a
groupedtable of those groups of T ^^^^^^^^^^^^^^^^^^ for which the result of
the<search condition> is true.
This is quite clear that the output of a HAVING clause is a "grouped
table" no matter whether the query uses GROUP BY or aggregates or not.
What that means is that neither the HAVING clause nor the targetlist
can use any ungrouped columns except within aggregate calls; that is,
select col from tab having 2>1
is in fact illegal per SQL spec, because col isn't a grouping column
(there are no grouping columns in this query).
What we are currently doing with this construct is pretending that it
means
select col from tab where 2>1
but it does not mean that according to the spec.
As I look into this, I find that several warty special cases in the
parser and planner arise from our misunderstanding of this point,
and could be eliminated if we enforced the spec's interpretation.
In particular this whole business of "moving HAVING into WHERE" is
wrong and should go away.
Comments? Can anyone confirm whether DB2 or other databases allow
ungrouped column references with HAVING?
regards, tom lane
Tom Lane wrote: > What that means is that neither the HAVING clause nor the targetlist > can use any ungrouped columns except within aggregate calls; that is, > > select col from tab having 2>1 > > is in fact illegal per SQL spec, because col isn't a grouping column > (there are no grouping columns in this query). [...] > Comments? Can anyone confirm whether DB2 or other databases allow > ungrouped column references with HAVING? Oracle does not allow such references. It issues "ORA-00979: not a GROUP BY expression" when you try to hand it such a reference. MS SQL Server does not allow such references either, yielding "columnname is invalid in the HAVING clause because it is not contained in either an aggregate function or the GROUP BY clause.". Can't comment about DB2. -- Kevin Brown kevin@sysexperts.com
Tom Lane <tgl@sss.pgh.pa.us> writes: > In particular this whole business of "moving HAVING into WHERE" is > wrong and should go away. It sort of seems like "select aggregate(col) from tab" with no GROUP BY clause is a bit of a special case. The consistent thing to do would be to return no records. It's only due to the special case that SQL returns a single record for this case. It seems like this special case is the only way to expose this difference between a WHERE clause and a HAVING clause with an aggregate-free expression. It seems like all that's needed is a simple flag on the Aggregate node that says whether to output a single record if there are no input records or to output no records. -- greg
>>Comments? Can anyone confirm whether DB2 or other databases allow >>ungrouped column references with HAVING? > > Oracle does not allow such references. It issues "ORA-00979: not a > GROUP BY expression" when you try to hand it such a reference. > > MS SQL Server does not allow such references either, yielding > "columnname is invalid in the HAVING clause because it is not > contained in either an aggregate function or the GROUP BY clause.". > > Can't comment about DB2. MySQL allows it: mysql> create table tab (col integer); Query OK, 0 rows affected (0.01 sec) mysql> select col from tab having 2 > 1; Empty set (0.00 sec) mysql> insert into tab values (1); Query OK, 1 row affected (0.00 sec) mysql> select col from tab having 2 > 1; +------+ | col | +------+ | 1 | +------+ 1 row in set (0.00 sec) Of course, that's not saying much! Chris
Christopher Kings-Lynne <chriskl@familyhealth.com.au> writes:
>>> Comments? Can anyone confirm whether DB2 or other databases allow
>>> ungrouped column references with HAVING?
> MySQL allows it:
A slightly tighter experiment shows that they treat HAVING like WHERE
in this case:
mysql> create table tab(col int);
Query OK, 0 rows affected (0.00 sec)
mysql> insert into tab values(1);
Query OK, 1 row affected (0.00 sec)
mysql> insert into tab values(2);
Query OK, 1 row affected (0.01 sec)
mysql> select col from tab having col>1;
+------+
| col |
+------+
| 2 |
+------+
1 row in set (0.00 sec)
I think it's fairly likely that they copied our misinterpretation ...
regards, tom lane
Greg Stark <gsstark@mit.edu> writes:
> It sort of seems like "select aggregate(col) from tab" with no GROUP BY clause
> is a bit of a special case. The consistent thing to do would be to return no
> records.
I don't think so. SQL99 defines this stuff in a way that might make you
feel better: it says that the presence of either HAVING or any aggregate
functions in the target list implies "GROUP BY ()", which is the case
that they identify as <grand total> in the <group by clause> syntax.
Basically this legitimizes the concept of turning the whole input table
into one group, which is what's really going on here. We get this right
in the case where it's driven by the appearance of aggregate functions,
but not when it's just driven by HAVING.
> It seems like all that's needed is a simple flag on the Aggregate node that
> says whether to output a single record if there are no input records or to
> output no records.
The implementation problem is that there *is* no aggregate node if there
are no aggregates. The definitional problem is that we are allowing
cases that are illegal per spec and are going to be difficult to
continue to support if we handle all the spec-required cases properly.
regards, tom lane
I wrote:
> This is quite clear that the output of a HAVING clause is a "grouped
> table" no matter whether the query uses GROUP BY or aggregates or not.
> What that means is that neither the HAVING clause nor the targetlist
> can use any ungrouped columns except within aggregate calls; that is,
> select col from tab having 2>1
> is in fact illegal per SQL spec, because col isn't a grouping column
> (there are no grouping columns in this query).
Actually, it's even more than that: a query with HAVING and no GROUP BY
should always return 1 row (if the HAVING succeeds) or 0 rows (if not).
If there are no aggregates, the entire from/where clause can be thrown
away, because it can have no impact on the result!
Would those of you with access to other DBMSes try this:
create table tab (col integer);
select 1 from tab having 1=0;
select 1 from tab having 1=1;
insert into tab values(1);
insert into tab values(2);
select 1 from tab having 1=0;
select 1 from tab having 1=1;
I claim that a SQL-conformant database will return 0, 1, 0, and 1 rows
from the 4 selects --- that is, the contents of tab make no difference
at all. (MySQL returns 0, 0, 0, and 2 rows, so they are definitely
copying our mistake...)
regards, tom lane
Tom Lane wrote: > Would those of you with access to other DBMSes try this: > > create table tab (col integer); > select 1 from tab having 1=0; > select 1 from tab having 1=1; > insert into tab values(1); > insert into tab values(2); > select 1 from tab having 1=0; > select 1 from tab having 1=1; > > I claim that a SQL-conformant database will return 0, 1, 0, and 1 rows > from the 4 selects --- that is, the contents of tab make no difference > at all. (MySQL returns 0, 0, 0, and 2 rows, so they are definitely > copying our mistake...) > > regards, tom lane From SQL server 2000 with a service pack, I get: zero rows from the first query (having 1=0); one row, col value 1, from second query (having 1=1); ...run inserts... zero rows from the third query (having 1=0); one row, col value 1, from forth query (having 1=1); K.
Tom Lane wrote: > > Would those of you with access to other DBMSes try this: > > create table tab (col integer); > select 1 from tab having 1=0; > select 1 from tab having 1=1; > insert into tab values(1); > insert into tab values(2); > select 1 from tab having 1=0; > select 1 from tab having 1=1; > > I claim that a SQL-conformant database will return 0, 1, 0, and 1 rows Not that this means much, but I'll mention it for the sake of completeness: SQLite 3.0.8 disallows all of the above SELECT statements: sqlite> create table tab (col integer); sqlite> select 1 from tab having 1=0; SQL error: a GROUP BY clause is required before HAVING sqlite> select 1 from tab having 1=1; SQL error: a GROUP BY clause is required before HAVING sqlite> insert into tab values(1); sqlite> insert into tab values(2); sqlite> select 1 from tab having 1=0; SQL error: a GROUP BY clause is required before HAVING sqlite> select 1 from tab having 1=1; SQL error: a GROUP BY clause is required before HAVING -- Michael Fuhr http://www.fuhr.org/~mfuhr/
On Thu, Mar 10, 2005 at 12:44:50PM -0500, Tom Lane wrote: > Would those of you with access to other DBMSes try this: DB2/LINUX 8.1.6 > create table tab (col integer); > select 1 from tab having 1=0; 1 ----------- 0 record(s) selected. > select 1 from tab having 1=1; 1 ----------- 1 1 record(s) selected. > insert into tab values(1); > insert into tab values(2); > select 1 from tab having 1=0; 1 ----------- 0 record(s) selected. > select 1 from tab having 1=1; 1 ----------- 1 1 record(s) selected. -johnnnnnnnn
On Thu, 10 Mar 2005 12:44:50 -0500, Tom Lane <tgl@sss.pgh.pa.us> wrote: > Would those of you with access to other DBMSes try this: > On informix 9.21.UC4 > create table tab (col integer); > select 1 from tab having 1=0; > returns no rows > select 1 from tab having 1=1; > returns no rows > insert into tab values(1); > insert into tab values(2); > select 1 from tab having 1=0; > returns no rows > select 1 from tab having 1=1; > returns 2 rows regards, Jaime Casanova
Tom Lane wrote:
> Would those of you with access to other DBMSes try this:
>
> create table tab (col integer);
> select 1 from tab having 1=0;
> select 1 from tab having 1=1;
> insert into tab values(1);
> insert into tab values(2);
> select 1 from tab having 1=0;
> select 1 from tab having 1=1;
>
> I claim that a SQL-conformant database will return 0, 1, 0, and 1 rows
> from the 4 selects --- that is, the contents of tab make no difference
> at all. (MySQL returns 0, 0, 0, and 2 rows, so they are definitely
> copying our mistake...)
Firebird 1.5.1 FreeBSD 5.3
Database: test
SQL> drop table tab;
SQL> create table tab (col integer);
SQL> select 1 from tab having 1=0;
SQL> select 1 from tab having 1=1;
============
1
SQL> insert into tab values(1);
SQL> insert into tab values(2);
SQL> select 1 from tab having 1=0;
SQL> select 1 from tab having 1=1;
============
1
SQL>
On Thu, 10 Mar 2005, Tom Lane wrote: > Would those of you with access to other DBMSes try this: > > create table tab (col integer); > select 1 from tab having 1=0; > select 1 from tab having 1=1; > insert into tab values(1); > insert into tab values(2); > select 1 from tab having 1=0; > select 1 from tab having 1=1; > > I claim that a SQL-conformant database will return 0, 1, 0, and 1 rows > from the 4 selects --- that is, the contents of tab make no difference > at all. (MySQL returns 0, 0, 0, and 2 rows, so they are definitely > copying our mistake...) DB2 (version 8.1) gives 0, 1, 0, 1. - Heikki
Results from Oracle 9.2.0.3 (9.2 patch 2)
===========
Connected to Oracle9i Release 9.2.0.3.0
Connected as system
SQL>
SQL> DROP TABLE TESTTAB;
DROP TABLE TESTTAB
ORA-00942: table or view does not exist
SQL> create table TESTtab (col integer);
Table created
SQL> select 1 as col from TESTtab having 1=0;
COL
----------
SQL> select 1 as col from TESTtab having 1=1;
COL
----------
SQL> insert into TESTtab values(1);
1 row inserted
SQL> insert into TESTtab values(2);
1 row inserted
SQL> select 1 as col from TESTtab having 1=0;
COL
----------
SQL> select 1 as col from TESTtab having 1=1;
COL
---------- 1 1
SQL> DROP TABLE TESTTAB;
Table dropped
SQL>
===========
On Thu, Mar 10, 2005 at 12:44:50PM -0500, Tom Lane wrote: > > Would those of you with access to other DBMSes try this: > > create table tab (col integer); > select 1 from tab having 1=0; > select 1 from tab having 1=1; > insert into tab values(1); > insert into tab values(2); > select 1 from tab having 1=0; > select 1 from tab having 1=1; > > I claim that a SQL-conformant database will return 0, 1, 0, and 1 rows > from the 4 selects --- that is, the contents of tab make no difference > at all. Sybase ASE version 12.5.2 returns 0, 0, 0, and 1 rows. A plain "select 1 from tab" returns zero rows when tab is empty. -- Mark Shewmaker mark@primefactor.com
Tom Lane wrote: > > Would those of you with access to other DBMSes try this: > > create table tab (col integer); > select 1 from tab having 1=0; > select 1 from tab having 1=1; > insert into tab values(1); > insert into tab values(2); > select 1 from tab having 1=0; > select 1 from tab having 1=1; > > I claim that a SQL-conformant database will return 0, 1, 0, and 1 rows MS SQL Server 2000 returns 0, 1, 0 and 1 rows correctly. Cheers, Gary.
>>select 1 from tab having 1=1;
>>
>
> returns 2 rows
I'm curious whats in those two rows... {{1} {1}} ?
On Wed, 2005-03-09 at 21:21 -0500, Tom Lane wrote: > Comments? Can anyone confirm whether DB2 or other databases allow > ungrouped column references with HAVING? In Sybase: 1> select 2 as id, max(myfield) from mytable where 2=1 2> go id ----------- ---------- 2 NULL (1 row affected) 1> select 2 as id, max(myfield) from mytable having 2=1 2> go id ----------- ---------- (0 rows affected) -- Mark Shewmaker mark@primefactor.com
> On Wed, 2005-03-09 at 21:21 -0500, Tom Lane wrote: > > Comments? Can anyone confirm whether DB2 or other databases allow > > ungrouped column references with HAVING? Mysql treats ungrouped columns as an assertion that those columns will all be equal for the group and it can pick an arbitrary one. Essentially it does an implicit "first(x) AS x". The expected use case is for things like: select count(*),a.* from a,bwhere a.pk = b.a_fkgroup by a.pk I've noticed quite frequently scenarios where this idiom would be very handy. I usually either end up rewriting the query to have nested subqueries so I can push the grouping into the subquery. This doesn't always work though and sometimes I end up listing several, sometimes dozens, of columns like "first(x) AS x" or else end up -- greg
On Sun, Mar 13, 2005 at 23:24:18 -0500, Greg Stark <gsstark@mit.edu> wrote: > > I've noticed quite frequently scenarios where this idiom would be very handy. > I usually either end up rewriting the query to have nested subqueries so I can > push the grouping into the subquery. This doesn't always work though and > sometimes I end up listing several, sometimes dozens, of columns like > "first(x) AS x" or else end up If someone did a naive implementation of first() and last() aggregates for 8.1, is that something that would likely be accepted?
Bruno Wolff III <bruno@wolff.to> writes: > If someone did a naive implementation of first() and last() aggregates > for 8.1, is that something that would likely be accepted? You mean like this? CREATE FUNCTION first_accum(anyelement,anyelement) RETURNS anyelement as 'select coalesce($1,$2)' LANGUAGE SQL;CREATE AGGREGATEfirst (BASETYPE=anyelement, SFUNC=first_accum, STYPE = anyelement); Though I suspect it would be faster as a native C implementation. -- greg
On Mon, Mar 14, 2005 at 00:35:32 -0500, Greg Stark <gsstark@mit.edu> wrote: > > Bruno Wolff III <bruno@wolff.to> writes: > > > If someone did a naive implementation of first() and last() aggregates > > for 8.1, is that something that would likely be accepted? > > You mean like this? > > CREATE FUNCTION first_accum(anyelement,anyelement) RETURNS anyelement as 'select coalesce($1,$2)' LANGUAGE SQL; > CREATE AGGREGATE first (BASETYPE=anyelement, SFUNC=first_accum, STYPE = anyelement); > > Though I suspect it would be faster as a native C implementation. Pretty much that idea. It just seemed odd to me that first and last weren't implemented, since they seemed to be simple and could be used in cases where max or min couldn't (because of no ordering) to do the same thing.
Greg Stark <gsstark@mit.edu> writes:
> On Wed, 2005-03-09 at 21:21 -0500, Tom Lane wrote:
>> Comments? Can anyone confirm whether DB2 or other databases allow
>> ungrouped column references with HAVING?
> Mysql treats ungrouped columns as an assertion that those columns will all be
> equal for the group and it can pick an arbitrary one. Essentially it does an
> implicit "first(x) AS x". The expected use case is for things like:
> select count(*),a.*
> from a,b
> where a.pk = b.a_fk
> group by a.pk
[ Your comment is completely unrelated to my question, but anyway: ]
Well, that query is actually legitimate per SQL99 (though not per SQL92)
if a.pk actually is a primary key. A correct implementation of SQL99
would deduce that the columns of A are all functionally dependent on
a.pk and not make you list them in GROUP BY. I dunno whether mysql goes
through that pushup or whether they just assume the user knows what he's
doing (though from what I know of their design philosophy I bet the
latter).
I'm not sure if we have a TODO item about working on the SQL99 grouping
rules, but I'd like to see us implement at least the simpler cases,
such as this one.
regards, tom lane
Bruno Wolff III <bruno@wolff.to> writes:
> If someone did a naive implementation of first() and last() aggregates
> for 8.1, is that something that would likely be accepted?
For the purpose that Greg is suggesting, these would have no advantage
over min() or max() --- since the system wouldn't know how to optimize
them --- and they'd be considerably less standard. So my inclination
would be to say it's a waste of effort.
regards, tom lane
On Mon, Mar 14, 2005 at 01:52:59 -0500, Tom Lane <tgl@sss.pgh.pa.us> wrote: > Bruno Wolff III <bruno@wolff.to> writes: > > If someone did a naive implementation of first() and last() aggregates > > for 8.1, is that something that would likely be accepted? > > For the purpose that Greg is suggesting, these would have no advantage > over min() or max() --- since the system wouldn't know how to optimize > them --- and they'd be considerably less standard. So my inclination > would be to say it's a waste of effort. The case I was thinking of were datatypes without a defined ordering where max and min wouldn't be usable. But if GROUP BY was going to changed to allow any columns if the primary key was used in the GROUP BY clause, I can't see any use for those functions.
Bruno Wolff III <bruno@wolff.to> writes: > The case I was thinking of were datatypes without a defined ordering > where max and min wouldn't be usable. But if GROUP BY was going to > changed to allow any columns if the primary key was used in the GROUP > BY clause, I can't see any use for those functions. Well any other case will basically be a another spelling for DISTINCT ON. Except DISTINCT ON only handles a limited range of cases. Basically DISTINCT ON is a special case of GROUP BY where the _only_ aggregate function you're allowed is first(). Handling the same cases using GROUP BY would let you mix other aggregate functions so where you have: select distinct on (x) x,y,zorder by x,y,z You can do the equivalent: select x, first(y), first(z)order by x,y,zgroup by x But you can also handle the more general case like: select x, first(y), first(z), avg(a), sum(s)order by x,y,zgroup by x I don't really care one way or the other about the "first" function per se. But it seems odd to have a feature to handle a special case of an existing much more general feature separately. It seems it would be more worthwhile to handle the general case of aggregate functions that don't need all the records to generate an answer, including first(), last(), min(), and max(). That would better handle the cases DISTINCT ON handles but also solve many other problems. -- greg
On Fri, 11 Mar 2005 10:37:13 +1300, Mark Kirkwood
<markir@coretech.co.nz> wrote:
>Firebird 1.5.1 FreeBSD 5.3
>[correct results]
Interbase 6.0:
SQL> create table tab (col integer);
SQL> select 1 from tab having 1=0;
SQL> select 1 from tab having 1=1;
============
0 <------- :-)
SQL> insert into tab values(1);
SQL> insert into tab values(2);
SQL> select 1 from tab having 1=0;
SQL> select 1 from tab having 1=1;
============
1
SQL>
ServusManfred
On 14 Mar 2005, Greg Stark wrote: > select distinct on (x) x,y,z > order by x,y,z > > You can do the equivalent: > > select x, first(y), first(z) > order by x,y,z > group by x > > But you can also handle the more general case like: > > select x, first(y), first(z), avg(a), sum(s) > order by x,y,z > group by x > > I don't really care one way or the other about the "first" function per se. The standard (sql2003) have what is called windows where one can do these things and much more. A window is like a group by, but you keep all rows in the result. This can be used to for example enumrate the rows within a window partition using ROW_NUMBER(). It can later can be used in a WHERE to select the top 3 rows in each window, or something like that. Here is an example that calculate the avg and sum for each window. It return all the rows (x values) in the window together with a row number (within the window) and the 2 aggregate results. In this case the aggregates will be the same for all rows in the partition but one can also get it to do a kind of of sliding window aggregate (for example the avarage of the row before and the row after the current row): SELECT ROW_NUMBER() OVER bar AS num, x, avg(a) OVER bar, sum (a) OVER bar FROM foo WINDOW bar AS PARTITION BY x ORDER BY x, y, z; and then one can put that whole thing as a subselect and just select the rows with num = 1. This doesn't mean that we don't want functions like first() and last(), they are also be useful. I just wanted to inform that with sql2003 one can write queries with the same effect as the above (but much more complicated, of course :-). ps. All I know about the window functions is from what I've read in the draft of the sql2003 standard. It's not the perfect way to learn about new features so I wont bet my life on that the above example works as is. If someone knows better I'd like to hear about it. ps2. I'd love to read a book that discusses the sql2003 (or even sql99) that explain features, give examples, and so on. But i guess the market don't want books that explain things that no database have implemented yet (Oracle have window functions but i've never used that). -- /Dennis Björklund
Dennis Bjorklund <db@zigo.dhs.org> writes: > The standard (sql2003) have what is called windows where one can do these > things and much more. OLAP functions would be very nice. But they're not the same thing. In fact that's precisely *why* they would be really nice. They allow you to do things that are not feasible to do with just plain old GROUP BY. But I stared at this spec for a while and it seemed like implementing it would be pretty hard. Like, I don't see any way to implement your query below without sorting every record in the group repeatedly for every record. That would be a lot of sorts. > SELECT ROW_NUMBER() OVER bar AS num, > x, > avg(a) OVER bar, > sum (a) OVER bar > FROM foo > WINDOW bar AS PARTITION BY x ORDER BY x, y, z; Note that as you said, this returns just as many records as are in the original table. The OLAP functions here are just regular functions, not aggregate functions -- albeit functions that use data from other records other than the one being output. DISTINCT ON is just another spelling for GROUP BY, it always outputs only one record per group. > ps2. I'd love to read a book that discusses the sql2003 (or even sql99) > that explain features, give examples, and so on. But i guess the market > don't want books that explain things that no database have implemented yet > (Oracle have window functions but i've never used that). DB2 has them too. Check out this DB2 documentation, it might be more helpful than the specs. http://publib.boulder.ibm.com/infocenter/rb63help/index.jsp?topic=/com.ibm.redbrick.doc6.3/sqlrg/sqlrg35.htm -- greg
On 14 Mar 2005, Greg Stark wrote: > > SELECT ROW_NUMBER() OVER bar AS num, > > x, > > avg(a) OVER bar, > > sum (a) OVER bar > > FROM foo > > WINDOW bar AS PARTITION BY x ORDER BY x, y, z; > > Note that as you said, this returns just as many records as are in the > original table. The OLAP functions here are just regular functions, not > aggregate functions They are aggregate functions, the avg() is a window aggregate function according to the standard. It runs over all values in the same partition. > -- albeit functions that use data from other records other > than the one being output. Yes, and not just one other record, but a number of them. Isn't that what aggregate functions are? Anyway, I just wanted to point to this area in the standard to tell others what are possible using standard constructs. It doesn't really help anything in this specific case. Pg will not have any of this implemented in the nearest future (I guess). There is always a chance that someone see the mail, get interested, learn about it and then implements it :-) -- /Dennis Björklund
Dennis Bjorklund <db@zigo.dhs.org> writes: > They are aggregate functions, the avg() is a window aggregate function > according to the standard. It runs over all values in the same partition. > > > -- albeit functions that use data from other records other > > than the one being output. > > Yes, and not just one other record, but a number of them. Isn't that what > aggregate functions are? Er, yeah. They're aggregate functions in that they consider many records as input. However they're not aggregate functions in that they produce an output for every record, as opposed to outputting only one value for a whole group. -- greg