Re: limit in subquery causes poor selectivity estimation

On lör, 2011-08-27 at 13:32 -0400, Tom Lane wrote:
> > EXPLAIN SELECT * FROM test1 WHERE sha1 in (SELECT sha1 FROM test2
> LIMIT 200);
>
> > Here, however, it has apparently not passed this knowledge through
> the
> > LIMIT.
>
> The LIMIT prevents the subquery from being flattened entirely, ie we
> don't have just "test1 SEMI JOIN test2" but "test1 SEMI JOIN (SELECT *
> FROM test2 LIMIT 200)". If you look at examine_variable in selfuncs.c
> you'll note that it punts for Vars coming from unflattened subqueries.
>
> > So what's up with that? Just a case of, we haven't thought about
> > covering this case yet, or are there larger problems?
>
> The larger problem is that if a subquery didn't get flattened, it's
> often because it's got LIMIT, or GROUP BY, or some similar clause that
> makes it highly suspect whether the statistics available for the table
> column are reasonable to use for the subquery outputs. It wouldn't be
> that hard to grab the stats for test2.sha1, but then how do you want
> to adjust them to reflect the LIMIT?

It turns out that this is a regression introduced in 8.4.8; the same
topic is also being discussed in

http://archives.postgresql.org/pgsql-performance/2011-08/msg00248.php

and

http://archives.postgresql.org/pgsql-general/2011-08/msg00995.php

This is the (previously posted) plan with 8.4.8:

QUERY PLAN
----------------------------------------------------------------------------------
Hash Join (cost=10.60..34.35 rows=500 width=31)
Hash Cond: (test1.sha1 = test2.sha1)
-> Seq Scan on test1 (cost=0.00..18.00 rows=1000 width=31)
-> Hash (cost=8.10..8.10 rows=200 width=32)
-> HashAggregate (cost=6.10..8.10 rows=200 width=32)
-> Limit (cost=0.00..3.60 rows=200 width=21)
-> Seq Scan on test2 (cost=0.00..18.01 rows=1001 width=21)

And this is the plan with 8.4.7:

QUERY PLAN
----------------------------------------------------------------------------------
Hash Join (cost=10.80..34.55 rows=200 width=31)
Hash Cond: (test1.sha1 = test2.sha1)
-> Seq Scan on test1 (cost=0.00..18.00 rows=1000 width=31)
-> Hash (cost=8.30..8.30 rows=200 width=32)
-> HashAggregate (cost=6.30..8.30 rows=200 width=32)
-> Limit (cost=0.00..3.80 rows=200 width=21)
-> Seq Scan on test2 (cost=0.00..19.01 rows=1001 width=21)

I liked the old one better. ;-)