空的LIKE对准备好的语句的性能影响

Question

I have set a GiST pg_trgm index on the name column of the files table.

The simplified query of the prepared statement looks like this:

SELECT * FROM files WHERE name LIKE $1;

The $1 param will be composed of % + user query + % . Since the input might also be an empty string this might result in %% .

Does an "empty" LIKE ( %% ) result in performance degradation? Should I build a new query in that case, or does it not matter?

Answer 1

Postgres 9.2 or later is generally smart enough to realize that the condition

WHERE name LIKE '%%'

is not selective and resorts to a sequential scan ignoring the GiST index - even with prepared statements. You do pay a small price for the useless condition, though.

In Postgres 9.1 or earlier I would build a separate query for the special case.

Compare the Notes section for the PREPARE statement in the manual for the versions 9.1 , 9.2 and 9.3 .

Verify yourself

Prepare the statement and run EXPLAIN ANALYZE to test:

PREPARE plan1 (text) AS
SELECT  * FROM file
WHERE   name LIKE $1;

EXPLAIN ANALYZE EXECUTE plan1('%123%');

EXPLAIN ANALYZE EXECUTE plan1('%%');

Plans are generally cached for the duration of the session.

Alternative query

Regardless of the version you are running, if you always perform a full text search (wildcards left and right), this query should be faster for a prepared statement:

SELECT * FROM files WHERE name LIKE ;

And pass the pattern without added wildcards ( % ), of course. This way, Postgres knows to expect a pattern enclosed in wildcards at planning time.

-> SQLfiddle demo.
Note the sequential scan for the empty LIKE and the performance difference between the two plans.
SQLfiddle varies a lot, depending on load etc. A single run may not be reliable. Better test in your environment and run each statement a couple of times to saturate cache and eliminate noise.