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best_inner_indexscan vs. reality

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-hackers(at)postgreSQL(dot)org
Subject: best_inner_indexscan vs. reality
Date: 2007-05-21 20:52:56
Message-ID: 26479.1179780776@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
I looked into the curious planner behavior described in this thread:
http://archives.postgresql.org/pgsql-performance/2007-05/msg00388.php
and after a bit of experimentation was able to duplicate it in the
regression database:

regression=# explain select * from int4_tbl a where f1 in (select hundred from tenk1 b);
                            QUERY PLAN                             
-------------------------------------------------------------------
 Nested Loop IN Join  (cost=0.00..30.20 rows=5 width=4)
   Join Filter: (a.f1 = b.hundred)
   ->  Seq Scan on int4_tbl a  (cost=0.00..1.05 rows=5 width=4)
   ->  Seq Scan on tenk1 b  (cost=0.00..458.00 rows=10000 width=4)
(4 rows)

regression=# set enable_bitmapscan TO 0;
SET
regression=# explain select * from int4_tbl a where f1 in (select hundred from tenk1 b);
                                      QUERY PLAN                                       
---------------------------------------------------------------------------------------
 Nested Loop IN Join  (cost=0.00..13.21 rows=5 width=4)
   ->  Seq Scan on int4_tbl a  (cost=0.00..1.05 rows=5 width=4)
   ->  Index Scan using tenk1_hundred on tenk1 b  (cost=0.00..242.00 rows=100 width=4)
         Index Cond: (b.hundred = a.f1)
(4 rows)

WTF?  How can disabling an unrelated plan type cause the thing to find a
cheaper plan than it found otherwise?

After some digging, the problem can be laid at the feet of
best_inner_indexscan, whose comment indeed foresees this issue:

 * best_inner_indexscan
 *	  Finds the best available inner indexscan for a nestloop join
 *	  with the given rel on the inside and the given outer_rel outside.
 *	  May return NULL if there are no possible inner indexscans.
 *
 * We ignore ordering considerations (since a nestloop's inner scan's order
 * is uninteresting).  Also, we consider only total cost when deciding which
 * of two possible paths is better --- this assumes that all indexpaths have
 * negligible startup cost.  (True today, but someday we might have to think
 * harder.)  Therefore, there is only one dimension of comparison and so it's
 * sufficient to return a single "best" path.

The "best" inner indexscan for this query is a bitmap scan with startup
cost of 5 and total cost of 170 (beating the plain indexscan's total
cost of 242).  However, for this IN join that's estimated as a worse
choice than the seqscan, precisely because of its startup cost.
We estimate a nestloop IN-join on the assumption that we'll stop
scanning after fetching one matching inner tuple; therefore, if there
are lots of potentially matching inner tuples, both a seqscan and an
indexscan look pretty cheap (less than 5 cost units to find the first
match), while the bitmap scan loses because of its startup cost.
Disabling bitmap scans allows the regular indexscan to be seen as
cheapest by best_inner_indexscan, and so it survives to be chosen as the
join partner.

Clearly, best_inner_indexscan's API is obsolete now that bitmap scans
exist.  What I'm inclined to do is make it cache and return both the
cheapest-total and cheapest-startup plans for any given combination of
rels.  (This should add only negligibly to its cost, since it's
enumerating all the possible paths anyway.)  To avoid useless effort
in match_unsorted_outer(), it should probably consider the
cheapest-startup only when doing JOIN_IN cases --- AFAICS there isn't
any other case where startup cost can outweigh total cost for the inside
of a nestloop.

I'm not sure if it's worth considering this issue for
best_appendrel_indexscan() --- in an Append situation it's not clear
that startup costs of individual subplans mean much of anything.

Comments?

Also, should this be backpatched?  Both 8.1 and 8.2 have got the issue;
prior releases no, since they didn't have bitmap scans.

			regards, tom lane

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