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Using PG 9.0 and given 2 queries (messageq_current is a view on the messageq_table): select entity_id from messageq_current where entity_id = 123456; select entity_id from messageq_current where incoming = true and inactive = false and staff_ty = 2 and staff_id = 2 order by entity_id desc limit 1; and 2 indexes (there are 15 indexes in total but they are left out here for brevity): messageq1: CREATE INDEX messageq1 ON messageq_table USING btree (entity_id); And messageq4: CREATE INDEX messageq4 ON messageq_table USING btree (inactive, staff_ty, staff_id, incoming, tran_dt); With the messageq1 index present, query 1 is very quick (0.094ms) and query 2 is very slow (241.515ms). If I remove messageq1 then query 2 uses messageq4 and is very quick (0.098ms) but then query 1 must use a different index and is therefore slower (> 5ms). So, to the Query plans: With messageq1: "Limit (cost=0.00..2670.50 rows=1 width=4) (actual time=241.481..241.481 rows=0 loops=1)" " Output: messageq_table.entity_id" " Buffers: shared hit=32 read=18870 written=12" " -> Index Scan Backward using messageq1 on prac_live_10112.messageq_table (cost=0.00..66762.53 rows=25 width=4) (actual time=241.479..241.479 rows=0 loops=1)" " Output: messageq_table.entity_id" " Filter: (messageq_table.incoming AND (NOT messageq_table.inactive) AND (messageq_table.staff_ty = 2) AND (messageq_table.staff_id = 2) AND (aud_status_to_flag(messageq_table.aud_status) = 1))" " Buffers: shared hit=32 read=18870 written=12" "Total runtime: 241.515 ms" Without messageq1: "Limit (cost=12534.45..12534.45 rows=1 width=4) (actual time=0.055..0.055 rows=0 loops=1)" " Output: messageq_table.entity_id" " Buffers: shared read=3" " -> Sort (cost=12534.45..12534.51 rows=25 width=4) (actual time=0.054..0.054 rows=0 loops=1)" " Output: messageq_table.entity_id" " Sort Key: messageq_table.entity_id" " Sort Method: quicksort Memory: 17kB" " -> Bitmap Heap Scan on prac_live_10112.messageq_table (cost=174.09..12534.32 rows=25 width=4) (actual time=0.043..0.043 rows=0 loops=1)" " Output: messageq_table.entity_id" " Recheck Cond: ((messageq_table.staff_ty = 2) AND (messageq_table.staff_id = 2))" " Filter: (messageq_table.incoming AND (NOT messageq_table.inactive) AND (aud_status_to_flag(messageq_table.aud_status) = 1))" " Buffers: shared read=3" " -> Bitmap Index Scan on messageq4 (cost=0.00..174.08 rows=4920 width=0) (actual time=0.040..0.040 rows=0 loops=1)" " Index Cond: ((messageq_table.inactive = false) AND (messageq_table.staff_ty = 2) AND (messageq_table.staff_id = 2) AND (messageq_table.incoming = true))" " Buffers: shared read=3" "Total runtime: 0.098 ms" Clearly the statistics are off somehow but I really don’t know where to start. Any help you can give me would be very much appreciated. Regards,
Russell Keane
INPS
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