From: pgsql-general-owner@postgresql.org [mailto:pgsql-general-owner@postgresql.org] On Behalf Of - - Sent: Monday, August 08, 2011 3:40 AM To: pgsql-general@postgresql.org Subject: Re: [GENERAL] Query with rightmost function does not use index
Here are the EXPLAIN ANALYZE outputs:
explain analyze select * from filter_item where filter_hash = MD5('');
QUERY PLAN ----------------------------------------------------------------------------------------------------------------------- Seq Scan on filter_item (cost=0.00..424644.96 rows=86108 width=49) (ac tual time=8177.807..12421.921 rows=77 loops=1) Filter: ((filter_hash)::text = 'd41d8cd98f00b204e9800998ecf8427e'::text) Total runtime: 12421.959 ms (3 rows)
explain analyze select * from filter_item where filter_hash = 'd41d8cd98f00b204e9800998ecf8427e'
QUERY PLAN -------------------------------------------------------------------------------------------------------------------------------------- Bitmap Heap Scan on filter_item (cost=77.92..6609.02 rows=3534 width=49) (actual time=0.055..0.100 rows=77 loops=1) Recheck Cond: (filter_hash = 'd41d8cd98f00b204e9800998ecf8427e'::bpchar) -> Bitmap Index Scan on filter_item__filter_hash (cost=0.00..77.04 rows=3534 width=0) (actual time=0.049..0.049 rows=77 loops=1) Index Cond: (filter_hash = 'd41d8cd98f00b204e9800998ecf8427e'::bpchar) Total runtime: 0.130 ms (5 rows)
The filter_hash index uses a “character(n)” data type – the ::bpchar. The second query is of unknown type and thus is converted to “character” and then used in the index. The first query use a function that outputs a “text”. Since the output type is known the left-side of the equals is casted to that known type. Since the index is one the “character” version of the filter_hash but the comparison requires a “text” version the index cannot be used. You would need to manually cast the result of the md5 function call to “character” in order to get the index usage; or convert the filter_hash column to text, the latter option probably being preferred.
It is not a bug, in cases of uncertainty the types of the value and the indexed field must be the same, but it could possibly be more user-friendly.
I’ll leave it to other to comment on whether this is different in more recent versions. Text-character are binary compatible and so it is not be unreasonable to assume, like you did, that indexes of one should be usable by the other.