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explicit casting required for index use

From: Reece Hart <reece(at)in-machina(dot)com>
To: "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org>,pgsql-general(at)postgresql(dot)org
Subject: explicit casting required for index use
Date: 2003-10-25 17:49:00
Message-ID: 1067104140.16297.125.camel@tallac (view raw, whole thread or download thread mbox)
Lists: pgsql-generalpgsql-performance
Here's the basic issue: PostgreSQL doesn't use indexes unless a query
criterion is of exactly the same type as the index type. This occurs
even when a cast would enable the use of an index and greatly improve
performance. I understand that casting is needed to use an index and
will therefore affect performance -- the part I don't understand is why
postgresql doesn't automatically cast query arguments to the column
type, thereby enabling indexes on that column.

I have a table that looks like this (extra cols, indexes, and fk
constraints removed):

        unison(at)csb=# \d paprospect2
                                        Table "unison.paprospect2"
           Column    |  Type   |                             Modifiers
         pseq_id     | integer | not null
         run_id      | integer | not null
         pmodel_id   | integer | not null
         svm         | real    |
        Indexes: paprospect2_search1 btree (pmodel_id, run_id, svm),

I often search for pseq_ids based on all of pmodel_id, run_id, and svm
threshold as below, hence the multi-column index.

Without an explicit cast of the svm criterion:

        unison(at)csb=# explain select pseq_id from paprospect2 where pmodel_id=8210 and run_id=1 and svm>=11;
         Index Scan using paprospect2_search2 on paprospect2  (cost=0.00..43268.93 rows=2 width=4)
           Index Cond: ((pmodel_id = 8210) AND (run_id = 1))
           Filter: (svm >= 11::double precision)

And with an explicit cast to real (the same as the column type and
indexed type):

        unison(at)csb=# explain select pseq_id from paprospect2 where pmodel_id=8210 and run_id=1 and svm>=11::real;
         Index Scan using paprospect2_search1 on paprospect2  (cost=0.00..6.34 rows=2 width=4)
           Index Cond: ((pmodel_id = 8210) AND (run_id = 1) AND (svm >= 11::real))

Note two things above: 1) The explicit cast greatly reduces the
predicted (and actual) cost. 2) The uncasted query eventually casts svm
to double precision, which seems odd since the column itself is real
(that is, it eventually does cast, but to the "wrong" type).

For small queries (returning ~10 rows), this is worth 100x in speed (9ms
v. 990ms... in absolute terms, no big deal). For larger result sets
(~200 rows), I've seen more like 1000x speed increases by using an
explicit cast. For the larger queries, this can mean seconds versus many

Having to explicitly cast criterion is very non-intuitive. Moreover, it
seems quite straightforward that PostgreSQL might incorporate casts (and
perhaps even function calls like upper() for functional indexes) into
its query strategy optimization. (I suppose functional indexes would
apply only to immutable fx only, but that's fine.)


Reece Hart, Ph.D.                       rkh(at)gene(dot)com,
Genentech, Inc.                         650/225-6133 (voice), -5389 (fax)
Bioinformatics and Protein Engineering
1 DNA Way, MS-93              
South San Francisco, CA  94080-4990     reece(at)in-machina(dot)com, GPG: 0x25EC91A0


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