Re: 7.4.7: strange planner decision

# dev(at)archonet(dot)com / 2005-07-13 12:57:31 +0100:
> Roman Neuhauser wrote:
> >Why does the planner want to crawl the table that has 5M rows instead of
> >the one
> >with 176k rows? Both tables are freshly vacuum-full-analyzed.
>
> Because you don't have an index on "base" for the files table.

I added one, ran vacuum full analyze fix.files, and:

callrec32=# \d fix.files
Table "fix.files"
Column | Type | Modifiers
--------+------------------------+-----------
dir | character varying(255) |
base | character varying(255) |
Indexes:
"base_storename_idx" btree (base, ((((dir)::text || '/'::text) || (base)::text)))
"ff_baseonly_idx" btree (base)
"ff_storename_idx" btree (((((dir)::text || '/'::text) || (base)::text)))

callrec32=# explain select fd.base from fix.dups fd join fix.files ff using (base);
QUERY PLAN
----------------------------------------------------------------------------
Hash Join (cost=5340.00..292675.06 rows=176161 width=44)
Hash Cond: (("outer".base)::text = ("inner".base)::text)
-> Seq Scan on files ff (cost=0.00..117301.58 rows=5278458 width=41)
-> Hash (cost=3436.60..3436.60 rows=176160 width=44)
-> Seq Scan on dups fd (cost=0.00..3436.60 rows=176160 width=44)
(5 rows)

Which is exactly what I expected. Using left prefix of a multicolumn
index normally works just fine, thank you.

http://www.postgresql.org/docs/7.4/static/indexes-multicolumn.html:

The query planner can use a multicolumn index for queries that involve
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
the leftmost column in the index definition plus any number of columns
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
listed to the right of it, without a gap. For example, an index on (a,
b, c) can be used in queries involving all of a, b, and c, or in queries
involving both a and b, or in queries involving only a

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