Tom Lane wrote:
John Meinel <[EMAIL PROTECTED]> writes:

... However, if I try to bundle this query up into a server side function, it runs very slow (10 seconds). I'm trying to figure out why, but since I can't run EXPLAIN ANALYZE inside a function, I don't really know what else to do.


A parameterized query inside a function is basically the same as a
PREPARE'd query with parameters at the SQL level.  So you can
investigate what's happening here with

        PREPARE foo(int) AS
                SELECT * FROM object WHERE id in (
                        SELECT id FROM data_t WHERE project_id = $1
                UNION SELECT ... ;

        EXPLAIN ANALYZE EXECUTE foo(48542);

I'm not sure where the problem is either, so please do send along the
results.

                        regards, tom lane

PS: pgsql-performance would be a more appropriate venue for this
discussion.

Well, I think I tracked the problem down to the fact that the column does not have a "not null" constraint on it. Here is a demonstration. Basically, I have 3 tables, tobjects, tdata, and tproject. tdata basically just links between tobjects and tproject, but isn't required to link to tproject. Yes, the real data has more columns, but this shows the problem.

jfmeinel=> \d tobjects
   Table "public.tobjects"
 Column |  Type   | Modifiers
--------+---------+-----------
 id     | integer | not null
Indexes:
    "tobjects_pkey" primary key, btree (id)

jfmeinel=> \d tproject
   Table "public.tproject"
 Column |  Type   | Modifiers
--------+---------+-----------
 id     | integer | not null
Indexes:
    "tproject_pkey" primary key, btree (id)

jfmeinel=> \d tdata
Table "public.tdata"
Column | Type | Modifiers
------------+---------+-----------
id | integer | not null
project_id | integer |
Indexes:
"tdata_pkey" primary key, btree (id)
"tdata_project_id_idx" btree (project_id)
Foreign-key constraints:
"tdata_id_fkey" FOREIGN KEY (id) REFERENCES tobjects(id) ON UPDATE CASCADE ON DELETE CASCADE
"tdata_project_id_fkey" FOREIGN KEY (project_id) REFERENCES
tproject(id) ON UPDATE CASCADE ON DELETE SET DEFAULT

jfmeinel=> select count(*) from tdata;
 count
--------
 545768

jfmeinel=> select count(*) - count(project_id) from tdata;
 ?column?
----------
      240

So tdata(project_id) is almost completely full, of the 540000+ entries, only 240 are null.


jfmeinel=> prepare myget(int) as select id from tdata jfmeinel-> where project_id = $1; PREPARE

jfmeinel=> explain analyze execute myget(30000);
                                                 QUERY PLAN
--------------------------------------------------------------------
 Seq Scan on tdata  (cost=0.00..9773.10 rows=181923 width=4)
        (actual time=1047.000..1047.000 rows=0 loops=1)
   Filter: (project_id = $1)
 Total runtime: 1047.000 ms

jfmeinel=> explain analyze select id from tdata where project_id = 30000;
                                                         QUERY PLAN

-------------------------------------------------------------------------
Index Scan using tdata_project_id_idx on tdata (cost=0.00..4.20 rows=1 width=4) (actual time=0.000..0.000 rows=0 loops =1)
Index Cond: (project_id = 30000)
Total runtime: 0.000 ms

So notice that when doing the actual select it is able to do the index query. But for some reason with a prepared statement, it is not able to do it.

Any ideas?

Since I only have the integers now, I can send the data to someone if they care to investigate it. It comes to 2.2M as a .tar.bz2, so obviously I'm not going to spam the list.

If I rewrite myget as:
prepare myget(int) as select id from tdata where project_id = 30000; it does the right thing again. So it's something about how a variable interacts with an indexed column with null values.

Note: I've tried creating a script that generates dummy data to show this problem and I have failed (it always performed the query correctly.)

But this test data definitely shows the problem. And yes, I've vacuum analyzed all over the place.

John
=:->

PS> I tested this on PostgreSQL 7.4.3, and it did not demonstrate this problem. I am using PostgreSQL 8.0.0beta2 (probably -dev1)

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