Re: Are bitmap index scans slow to start?

[Marc Mamin <M.Mamin@xxxxxxxxxxxx>]

>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:

WHERE session_id = 27 AND session_id%4 =27%4

regards,

Marc Mamin