I just created partitioned table, n_traf, sliced by month (n_traf_y2007m01, n_traf_y2007m02... and so on, see below). They are indexed by 'date_time' column. Then I populate it (last value have date 2007-08-...) and do VACUUM ANALYZE ON n_traf_y2007... all of it.
Now I try to select latest value (ORDER BY date_time LIMIT 1), but Postgres produced the ugly plan:
=# explain SELECT * FROM n_traf ORDER BY date_time DESC LIMIT 1; QUERY PLAN --------------------------------------------------------------------------------------------------------- Limit (cost=824637.69..824637.69 rows=1 width=32) -> Sort (cost=824637.69..838746.44 rows=5643499 width=32) Sort Key: public.n_traf.date_time -> Result (cost=0.00..100877.99 rows=5643499 width=32) -> Append (cost= 0.00..100877.99 rows=5643499 width=32) -> Seq Scan on n_traf (cost=0.00..22.30 rows=1230 width=32) -> Seq Scan on n_traf_y2007m01 n_traf (cost=0.00..22.30 rows=1230 width=32) -> Seq Scan on n_traf_y2007m02 n_traf (cost=0.00..22.30 rows=1230 width=32) -> Seq Scan on n_traf_y2007m03 n_traf (cost=0.00..22.30 rows=1230 width=32) -> Seq Scan on n_traf_y2007m04 n_traf (cost=0.00..1.01 rows=1 width=32) -> Seq Scan on n_traf_y2007m05 n_traf (cost=0.00..9110.89 rows=509689 width=32) -> Seq Scan on n_traf_y2007m06 n_traf (cost= 0.00..32003.89 rows=1790489 width=32) -> Seq Scan on n_traf_y2007m07 n_traf (cost=0.00..33881.10 rows=1895510 width=32) -> Seq Scan on n_traf_y2007m08 n_traf (cost= 0.00..25702.70 rows=1437970 width=32) -> Seq Scan on n_traf_y2007m09 n_traf (cost=0.00..22.30 rows=1230 width=32) -> Seq Scan on n_traf_y2007m10 n_traf (cost=0.00..22.30 rows=1230 width=32) -> Seq Scan on n_traf_y2007m11 n_traf (cost=0.00..22.30 rows=1230 width=32) -> Seq Scan on n_traf_y2007m12 n_traf (cost=0.00..22.30 rows=1230 width=32) (18 rows)
Why it no uses indexes at all? -------------------------------------------
I'm no expert but I'd guess that the the planner doesn't know which partition holds the latest time so it has to read them all.
