Hello, We are using PostgreSQL 9.0.3, compiled by Visual C++ build 1500, 32-bit, installed on Windows 2003 R2 32-bit. We have an ‘aisposition’ table used for a GPS tracking application, containing ~30 million rows and a number of indexes. Two of these are: idx_receiveddatetime: indexes aisposition(receiveddatetime timestamp) idx_userid_receiveddatetime: indexes aisposition(userid int4 desc, receiveddatetime timestamp desc) The problem we get is that the following query is taking many minutes to run: select * from aisposition where userid = 311369000 order by userid desc, receiveddatetime desc limit 1 When we ‘EXPLAIN’ this query, PostgreSQL says it is using the index idx_receiveddatetime. The way the application is designed means that in virtually all cases the query will have to scan a very long way into idx_receiveddatetime to find the first record where userid = 311369000. If however we delete the idx_receiveddatetime index, the query uses the idx_userid_receiveddatetime index, and the query only takes a few milliseconds. The EXPLAIN ANALYZE output with idx_receiveddatetime in place is: Limit (cost=0.00..1.30 rows=1 width=398) (actual time=1128097.540..1128097.541 rows=1 loops=1) -> Index Scan Backward using idx_receiveddatetime on aisposition (cost=0.00..2433441.05 rows=1875926 width=398) (actual time=1128097.532..1128097.532 rows=1 loops=1) Filter: (userid = 311369000) Total runtime: 1128097.609 ms And with that index deleted: Limit (cost=0.00..4.01 rows=1 width=398) (actual time=60.633..60.634 rows=1 loops=1) -> Index Scan using idx_userid_receiveddatetime on aisposition (cost=0.00..7517963.47 rows=1875926 width=398) (actual time=60.629..60.629 rows=1 loops=1) Index Cond: (userid = 311369000) Total runtime: 60.736 ms We would obviously prefer PostgreSQL to use the idx_userid_receiveddatetime index in all cases, because we know that this will guarantee results in a timely manner, whereas using idx_receiveddatetime will usually require a scan of much of the table and our application will not work. What are we doing wrong? Cheers now, John |
