I have a problem with partitioning and I'm wondering if anyone can provide some insight. I'm trying to find the max value of a column across multiple partitions. The query against the partition set is quite slow while queries against child partitions is very fast!
I setup a basic Range Partition table definition. A parent table: Data { dataID, sensorID, value, ts } child tables Data_YYYY_WEEKNO { dataID, sensorID, value, ts} inherited from Data Each child tables has a primary key index on dataID and a composite index on (sensorID, ts). Each child has check constraints for the week range identified in the table name (non overlapping) I want to perform a simple operation: select the max ts (timestamp) giving a sensorID. Given my indexs on the table, this should be a simple and fast operation. DB=# EXPLAIN ANALYZE select max(ts) from "Data" where valid=true and "sensorID"=8293 ; -------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Aggregate (cost=334862.92..334862.93 rows=1 width=8) (actual time=85183.381..85183.383 rows=1 loops=1) -> Append (cost=2.30..329397.68 rows=2186096 width=8) (actual time=1.263..76592.755 rows=2205408 loops=1) -> Bitmap Heap Scan on "Data" (cost=2.30..8.84 rows=3 width=8) (actual time=0.027..0.027 rows=0 loops=1) Recheck Cond: ("sensorID" = 8293) Filter: valid -> Bitmap Index Scan on "def_data_sensorID_ts" (cost=0.00..2.30 rows=6 width=0) (actual time=0.021..0.021 rows=0 loops=1) Index Cond: ("sensorID" = 8293) -> *Index Scan using "Data_2008_01_sensorID_ts_index" on "Data_2008_01" "Data"* (cost=0.00..4.27 rows=1 width=8) (actual time=0.014..0.014 rows=0 loops=1) Index Cond: ("sensorID" = 8293) Filter: valid -> *Bitmap Heap Scan on "Data_2008_02" "Data"* (cost=3.01..121.08 rows=98 width=8) (actual time=0.017..0.017 rows=0 loops=1) Recheck Cond: ("sensorID" = 8293) Filter: valid -> Bitmap Index Scan on "Data_2008_02_sensorID_ts_index" (cost=0.00..2.99 rows=98 width=0) (actual time=0.011..0.011 rows=0 loops=1) Index Cond: ("sensorID" = 8293) . . . (omitted a list of all partitions with same as data above) . Total runtime: 85188.694 ms When I query against a specific partition: DB=# EXPLAIN ANALYZE select max(ts) from "Data_2008_48" where valid=true and "sensorID"=8293 ; QUERY PLAN --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Result (cost=0.10..0.11 rows=1 width=0) (actual time=3.830..3.832 rows=1 loops=1) InitPlan -> Limit (cost=0.00..0.10 rows=1 width=8) (actual time=3.817..3.819 rows=1 loops=1) -> Index Scan Backward using "Data_2008_48_sensorID_ts_index" on "Data_2008_48" (cost=0.00..15304.55 rows=148959 width=8) (actual time=3.813..3.813 rows=1 loops=1) Index Cond: ("sensorID" = 8293) Filter: ((ts IS NOT NULL) AND valid) Total runtime: 0.225 ms The query plan against the child partition makes sense - Uses the index to find the max value. The query plan for the partitions uses a combination of bitmap heap scans and index scans. Why would the query plan choose to use a bitmap heap scan after bitmap index scan or is that the best choice? (what is it doing?) and what can I do to speed up this query? As a sanity check I did a union query of all partitions to find the max(ts). My manual union query executed in 13ms vs the query against the parent table that was 85,188ms!!!. Greg Jaman