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有一个测试表是这样的:Postgres没有使用索引超过一定的行数限制?
testdb=> \d test_table
Table "public.test_table"
Column | Type | Modifiers
----------------------------+-----------------------------+-----------------------------------------------------------
id | integer | not null default nextval('test_table_id_seq'::regclass)
phone | character varying(15) | not null
comment | text |
timestamp | timestamp without time zone | not null
Indexes:
"test_table_pkey" PRIMARY KEY, btree (id)
"i_test_substr" btree ("substring"(phone::text, 1, 1), "timestamp" DESC)
Triggers:
有了这样一个指标:
testdb=> create index i_test_substr ON test_table (substring(phone, 1, 1), timestamp desc);
这些查询将使用该索引下的203行的限制进行排序,但排序内存204行以上。
任何人都可以解释这种行为吗?
testdb=> explain analyze select * from test_table where substring(phone,1 ,1) = '2' order by timestamp desc limit 203;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=0.42..709.52 rows=203 width=202) (actual time=0.043..0.194 rows=203 loops=1)
-> Index Scan using i_test_substr on test_table (cost=0.42..1146.16 rows=328 width=202) (actual time=0.041..0.170 rows=203 loops=1)
Index Cond: ("substring"((phone)::text, 1, 1) = '2'::text)
Total runtime: 0.249 ms
(4 rows)
testdb=> explain analyze select * from test_table where substring(phone,1 ,1) = '2' order by timestamp desc limit 204;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=711.74..712.25 rows=204 width=202) (actual time=7.655..7.681 rows=204 loops=1)
-> Sort (cost=711.74..712.56 rows=328 width=202) (actual time=7.653..7.664 rows=204 loops=1)
Sort Key: "timestamp"
Sort Method: top-N heapsort Memory: 53kB
-> Bitmap Heap Scan on test_table (cost=10.96..698.03 rows=328 width=202) (actual time=1.340..5.010 rows=11514 loops=1)
Recheck Cond: ("substring"((phone)::text, 1, 1) = '2'::text)
-> Bitmap Index Scan on i_test_substr (cost=0.00..10.88 rows=328 width=0) (actual time=1.217..1.217 rows=11514 loops=1)
Index Cond: ("substring"((phone)::text, 1, 1) = '2'::text)
Total runtime: 7.746 ms
(9 rows)
请附上您的表格和查询行数的粒度没有限制+解释分析没有限制 –
'vacuum','analyse','reindex'没有帮助吗? – devanand
这就是优化器的工作原理。请参阅Bruce Momjan关于此的幻灯片,网址为https://www.google.fr/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwjCl9G05JXLAhVJDxoKHRfCAQcQFgggMAA&url=https%3A%2F%2Fmomjian.us%2Fmain %2Fwritings%2Fpgsql%2Foptimizer.pdf&USG = AFQjCNGDzCTCJt0n7BTrr_nqgfZIQMAUWQ&SIG2 = k_gYi_rSo3J-SvSel1saqA – greg