I have a table that includes two text columns t1 and t2, and a composite index on these columns. When issuing a query of the following form:
SELECT * FROM test WHERE t1 = 'X' and t2 = ANY(ARRAY['Y1', 'Y2', ..])
I have observed that it will use the index and have reasonable performance if the whole query is passed as a single big string. However if it is parameterised (I'm using Npgsql) it switches to doing a sequence scan and performs terribly.
The table contains approx 2.3 million records and the query matches about 20k records (i.e. there are 20k 'Y' values in the array).
I have experimented with changing work_mem, preparing the statement in advance, ANALYZEing the table, none of these change the behavior.
Why does the query planner choose to ignore the index when the command is parameterised?
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