>Rebuilding the index might help, as it would put all the leaf pages holding values for session_id=27 adjacent to each other, so they would read from disk faster. But with a name like >"session_id", I don't know how long such clustering would last though.
>If I'm right about the index disk-read time, then switching to a plain index scan rather than a bitmap index scan would make no difference--either way the data has to come off the disk.
>>I'd prefer a >>strategy that allowed fast performance the first time, rather than slow the >>first time and extremely fast subsequently.
Hello,
if the index is only used to locate rows for single session_id, you may consider split it in a set of partial indexes.
e.g. create index i_0 on foo where session_id%4 =0; create index i_1 on foo where session_id%4 =1; create index i_2 on foo where session_id%4 =2; create index i_3 on foo where session_id%4 =3;
(can be built in parallel using separate threads)
Then you will have to ensure that all your WHERE clauses also contain the index condition: