我注意到一些奇怪的/奇怪的:PostgreSQL查詢不使用生產索引
在開發/生產完全相同的查詢不使用相同的查詢路徑。特別是,開發版本使用的是在生產中省略的索引(有利於seqscan)。
唯一真正的區別是數據集的生產量要大得多 - 索引大小爲1034 MB,生產中爲29 MB。 PostgreSQL會放棄使用索引,如果它們(或表)太大?
編輯:EXPLAIN ANALYZE
兩個查詢:
發展:
Limit (cost=41638.15..41638.20 rows=20 width=154) (actual time=159.576..159.581 rows=20 loops=1)
-> Sort (cost=41638.15..41675.10 rows=14779 width=154) (actual time=159.575..159.577 rows=20 loops=1)
Sort Key: (sum(scenario_ad_group_performances.clicks))
Sort Method: top-N heapsort Memory: 35kB
-> GroupAggregate (cost=0.00..41244.89 rows=14779 width=154) (actual time=0.040..151.535 rows=14197 loops=1)
-> Nested Loop Left Join (cost=0.00..31843.75 rows=93800 width=154) (actual time=0.022..82.509 rows=50059 loops=1)
-> Merge Left Join (cost=0.00..4203.46 rows=14779 width=118) (actual time=0.017..27.103 rows=14197 loops=1)
Merge Cond: (scenario_ad_groups.id = scenario_ad_group_vendor_instances.ad_group_id)
-> Index Scan using scenario_ad_groups_pkey on scenario_ad_groups (cost=0.00..2227.06 rows=14779 width=114) (actual time=0.009..12.085 rows=14197 loops=1)
Filter: (scenario_id = 22)
-> Index Scan using index_scenario_ad_group_vendor_instances_on_ad_group_id on scenario_ad_group_vendor_instances (cost=0.00..1737.02 rows=27447 width=8) (actual time=0.007..7.021 rows=16528 loops=1)
Filter: (ad_vendor_id = ANY ('{1,2,3}'::integer[]))
-> Index Scan using index_ad_group_performances_on_vendor_instance_id_and_date on scenario_ad_group_performances (cost=0.00..1.73 rows=11 width=44) (actual time=0.002..0.003 rows=3 loops=14197)
Index Cond: ((vendor_instance_id = scenario_ad_group_vendor_instances.id) AND (date >= '2012-02-01'::date) AND (date <= '2012-02-28'::date))
Total runtime: 159.710 ms
生產:
Limit (cost=822401.35..822401.40 rows=20 width=179) (actual time=21279.547..21279.591 rows=20 loops=1)
-> Sort (cost=822401.35..822488.42 rows=34828 width=179) (actual time=21279.543..21279.560 rows=20 loops=1)
Sort Key: (sum(scenario_ad_group_performances.clicks))
Sort Method: top-N heapsort Memory: 33kB
-> GroupAggregate (cost=775502.60..821474.59 rows=34828 width=179) (actual time=19126.783..21226.772 rows=34495 loops=1)
-> Sort (cost=775502.60..776739.48 rows=494751 width=179) (actual time=19125.902..19884.164 rows=675253 loops=1)
Sort Key: scenario_ad_groups.id
Sort Method: external merge Disk: 94200kB
-> Hash Right Join (cost=25743.86..596796.70 rows=494751 width=179) (actual time=1155.491..16720.460 rows=675253 loops=1)
Hash Cond: (scenario_ad_group_performances.vendor_instance_id = scenario_ad_group_vendor_instances.id)
-> Seq Scan on scenario_ad_group_performances (cost=0.00..476354.29 rows=4158678 width=44) (actual time=0.043..8949.640 rows=4307019 loops=1)
Filter: ((date >= '2012-02-01'::date) AND (date <= '2012-02-28'::date))
-> Hash (cost=24047.72..24047.72 rows=51371 width=143) (actual time=1123.896..1123.896 rows=34495 loops=1)
Buckets: 1024 Batches: 16 Memory Usage: 392kB
-> Hash Right Join (cost=6625.90..24047.72 rows=51371 width=143) (actual time=92.257..1070.786 rows=34495 loops=1)
Hash Cond: (scenario_ad_group_vendor_instances.ad_group_id = scenario_ad_groups.id)
-> Seq Scan on scenario_ad_group_vendor_instances (cost=0.00..11336.31 rows=317174 width=8) (actual time=0.020..451.496 rows=431770 loops=1)
Filter: (ad_vendor_id = ANY ('{1,2,3}'::integer[]))
-> Hash (cost=5475.55..5475.55 rows=34828 width=139) (actual time=88.311..88.311 rows=34495 loops=1)
Buckets: 1024 Batches: 8 Memory Usage: 726kB
-> Bitmap Heap Scan on scenario_ad_groups (cost=798.20..5475.55 rows=34828 width=139) (actual time=4.451..44.065 rows=34495 loops=1)
Recheck Cond: (scenario_id = 276)
-> Bitmap Index Scan on index_scenario_ad_groups_on_scenario_id (cost=0.00..789.49 rows=34828 width=0) (actual time=4.232..4.232 rows=37006 loops=1)
Index Cond: (scenario_id = 276)
Total runtime: 21306.697 ms
該指數在您的生產環境中可能沒有足夠的選擇性 – 2012-02-28 03:14:30
「不夠選擇性」是什麼意思? – vonconrad 2012-02-28 03:30:40
如果你有一個帶有索引的布爾列,其中99%的數據是'false',那麼掃描索引然後掃描表格的99%,而不是掃描整個表格就沒有多大的意義了。對於「錯誤」值,索引不夠有選擇性。 – aib 2012-02-28 04:55:42