Re: Slow query with sub-select

[- - <loh.law@xxxxxxxxxxx>]

> - - <loh.law@xxxxxxxxxxx> writes:
> > The weird thing is that before I updated my server the query was about 5 times faster.
> > I've googled and I think the problem lies with the under-estimation of the query planner about the number of rows in the nested table.I will be trying the 'set enable_seqscan = false' solution to see if that'll improve.
>
> You evidently already do have that turned off. I'd suggest reverting
> that change (ie, allow seqscan) and instead increase work_mem enough
> so that the hash join can work without spilling to disk. This query
> is a perfect example of where indexes do not help, and trying to force
> them to be used makes things slower not faster.
>
> regards, tom lane

< div>

I have switched on seqscan and increased work_mem to 1GB ... but no luck so far.

The version I'm using is PostgreSQL 8.4.8 on i686-pc-linux-gnu, compiled by GCC gcc-4.5.real (Ubuntu/Linaro 4.5.2-8ubuntu4) 4.5.2, 32-bit

Before that I used an earlier minor version (8.4.x - I don't remember what x is but it was the one packaged in the version before Ubuntu Natty).

These are the relevant schemas.

< div>CREATE TABLE q (

  mid VARCHAR(10) NOT NULL

  ...

);

CREATE TABLE t (

  mid VARCHAR(10) NOT NULL PRIMARY KEY

  ...

);

I would like to count rows in q whose mid does not exist in t.

This is the query I used.

SELECT COUNT(*) FROM q

      WHERE NOT EXISTS (SELECT 1

                          FROM t

                         WHERE t.mid = q.mid);

Based on my understanding, I believe t he query will loop through each row in q (which has about 500m rows) and for each row it will check a one-to-one mapping against t (which has about 3m rows) by using an index scan on t (mid).

However, the EXPLAIN outputs for seqscan = on and seqscan = off, respectively, seem to indicate that it is not a one-to-one mapping of t.mid and q.mid.

I then switched the comparison operator in the where clause as follows:

SELECT COUNT(*) FROM q

      WHERE NOT EXISTS (SELECT 1

                          FROM t

                         WHERE q.mid = t.mid);

As there is no index on q (mid) this type of query should take a considerably longer time.  However, the EXPLAIN outputs seem to be the same.  

Here they are:

With seqscan = on

&nbs p;                                    QUERY PLAN                                      

-------------------------------------------------------------------------------------

 Aggregate  (cost=18566199.92..18566199.93 rows=1 width=0)

   ->  Hash Anti Join  (cost=747023.15..18566199.91 rows=1 width=0)

         Hash Cond: ((q.mid)::text = (t.mid)::text)

         ->  Seq Scan on q  (cost=0.00..11451989.24 rows=565972224 width=10)

         ->  Hash  (cost=701775.29..701775.29 rows=3619829 width=10)

               ->  Seq Scan on t  (cost=0.00..701775.29 rows=3619829 width=10)

(6 rows)

With seqscan = off

                                              QUERY PLAN                                               

-------------------------------------------------------------------------------------------------------

 Aggregate  (cost=10024599995.73..10024599995.74 rows=1 width=0)

   ->  Hash Anti Join  (cost=10006780818.96..10024599995.72 rows=1 width=0)

         Hash Cond: ((q.mid)::text = (t.mid)::text)

         ->  Seq Scan on q  (cost=1000000 0000.00..10011451989.24 rows=565972224 width=10)

         ->  Hash  (cost=6735571.10..6735571.10 rows=3619829 width=10)

               ->  Index Scan using t_pkey on t  (cost=0.00..6735571.10 rows=3619829 width=10)

(6 rows)

Any help is greatly appreciated as this problem has been depressing me for two weeks.