Postgres不使用索引，即使返回少于5％的行
[英]Postgres does not use an index even less than 5% rows are returned
问题描述
I have a postgres table with the following structure: 
我有一个具有以下结构的postgres表：
+---------+-------------+-------------+----------+---------+---------+
| id | timestamp | numvalues | text1 | text2 | text3 |
+---------+-------------+-------------+----------+---------+---------+
|abcd12344| 4124135 |[1,2,53,1241]| apple | banana | papaya |
+---------+-------------+-------------+----------+---------+---------+
id - random alphanumeric value<br>
timestamp - epoch timestamp<br>
numvalues - array of integers<br>
text(n) - text values
The table has about 150 million rows. 该表约有1.5亿行。
I make an inner query to get the nth percentile of the data ordered on one of the values in the array. 我进行内部查询，以获取按数组值之一排序的数据的第n个百分点。 Then, I need to get the averages of several values from the array. 然后，我需要从数组中获取多个值的平均值。 The length of the array is around 31. 数组的长度约为31。
SELECT AVG(COALESCE(numvalues[2], 0))::NUMERIC(10,0), AVG(COALESCE(numvalues[3], 0))::NUMERIC(10,0)...AVG(COALESCE(numvalues[12], 0))::NUMERIC(10,0)
FROM (SELECT timestamp, numvalues, ntile(100)
OVER (ORDER BY numvalues[1]) pval FROM tablename WHERE timestamp >= somevalue and timestamp <= somevalue) innertable
WHERE pval >= x and pval <= y;
This returns about 7 million rows out of 150 million which is about 5% of the table. 这将返回1.5亿行中的约700万行，约占表的5％。 However, it does not use the index. 但是，它不使用索引。 Running an EXPLAIN ANALYZE shows that it uses a Seq Scan instead, even when enable_seqscan is set to off. 即使将enable_seqscan设置为off，运行EXPLAIN ANALYZE也会显示它使用Seq扫描代替。
However, a similar query : 但是，类似的查询：
SELECT text1, count(distinct(id))
FROM (SELECT timestamp, id, text1, numvalues, ntile(100)
OVER (ORDER BY numvalues[1]) pval FROM tablename WHERE timestamp >= somevalue and timestamp <= somevalue) innertable
WHERE pval >= x and pval <= y GROUP BY text1;
does use the index. 确实使用索引。
The index is on the timestamp column 索引在时间戳列上
The results of EXPLAIN ANALYZE: EXPLAIN ANALYZE的结果：
explain analyze select text1, count(distinct(id))
from (select timestamp, text1, numvalues, id, ntile(100) over (order by numvalues[1]) pval from table where timestamp >= 1431100800 and timestamp <= 1431108000 and numvalues[1] NOTNULL) innertable
where pval >= 90 and pval <= 90 group by text1;
QUERY PLAN
--------------------------------------------------------------------------------
GroupAggregate (cost=4554118.06..4554641.66 rows=1 width=28) (actual time=218641.221..219051.984 rows=20 loops=1)
-> Sort (cost=4554118.06..4554292.59 rows=69812 width=28) (actual time=218640.546..218728.294 rows=71441 loops=1)
Sort Key: innertable.text1
Sort Method: quicksort Memory: 8654kB
-> Subquery Scan on innertable (cost=4094722.75..4548501.27 rows=69812 width=28) (actual time=216502.946..218521.666 rows=71441 loops=1)
Filter: ((innertable.pval >= 90) AND (innertable.pval <= 90))
Rows Removed by Filter: 7072674
-> WindowAgg (cost=4094722.75..4339065.03 rows=13962416 width=118) (actual time=202276.333..211374.235 rows=7144115 loops=1)
-> Sort (cost=4094722.75..4129628.79 rows=13962416 width=118) (actual time=164912.487..190272.316 rows=7144115 loops=1)
Sort Key: (table.numvalues[9])
Sort Method: external merge Disk: 1387704kB
-> Index Scan using table_time_idx on table (cost=0.57..1578710.87 rows=13962416 width=118) (actual time=0.124..141014.505 rows=7144115 loops=1)
Index Cond: (("timestamp" >= 1431100800) AND ("timestamp" <= 1431108000))
Filter: (numvalues[1] IS NOT NULL)
Rows Removed by Filter: 7090075
Total runtime: 219340.709 ms
(16 rows)
explain analyze select avg(coalesce(numvalues[9], 0))::NUMERIC(10,0) mean9, avg(coalesce(numvalues[30],0))::NUMERIC(10,0), avg(coalesce(numvalues[8],0))::NUMERIC(10,0) - avg(coalesce(numvalues[30], 0))::NUMERIC(10,0) mean0, avg(coalesce(numvalues[10],0))::NUMERIC(10,0) - avg(coalesce(numvalues[8], 0))::NUMERIC(10,0) mean1, avg(coalesce(numvalues[14],0))::NUMERIC(10,0) - avg(coalesce(numvalues[10], 0))::NUMERIC(10,0) mean2, avg(coalesce(numvalues[13],0))::NUMERIC(10,0) - avg(coalesce(numvalues[14], 0))::NUMERIC(10,0) mean3, avg(coalesce(numvalues[9],0))::NUMERIC(10,0) - avg(coalesce(numvalues[13], 0))::NUMERIC(10,0) mean4
from (select timestamp, id, numvalues, ntile(100) over (order by numvalues[1] ) pval from table where timestamp >= 1431093600.00 and timestamp <= 1431100800.00 and numvalues[9] NOTNULL) innerTable
where pval >= 90.00 and pval <= 90.00 ;
QUERY PLAN
--------------------------------------------------------------------------------
Aggregate (cost=12662077.32..12662077.37 rows=1 width=82) (actual time=650343.769..650343.770 rows=1 loops=1)
-> Subquery Scan on innertable (cost=12634854.54..12661968.84 rows=3615 width=82) (actual time=647745.962..650232.725 rows=71441 loops=1)
Filter: (((innertable.pval)::numeric >= 90.00) AND ((innertable.pval)::numeric <= 90.00))
Rows Removed by Filter: 7072674
-> WindowAgg (cost=12634854.54..12647507.88 rows=723048 width=248) (actual time=632388.293..642338.237 rows=7144115 loops=1)
-> Sort (cost=12634854.54..12636662.16 rows=723048 width=248) (actual time=599893.771..617413.102 rows=7144115 loops=1)
Sort Key: (table.numvalues[9])
Sort Method: external merge Disk: 3214248kB
-> Seq Scan on req_p0swajch2t (cost=0.00..12480460.48 rows=723048 width=248) (actual time=0.041..575423.062 rows=7144115 loops=1)
Filter: ((numvalues[1] IS NOT NULL) AND (("timestamp")::numeric >= 1431100800.00) AND (("timestamp")::numeric <= 1431108000.00))
Rows Removed by Filter: 138191935
Total runtime: 650785.126 ms
(12 rows)
Can anyone help me out with why Postgres would use an index for one query and not the other? 谁能为我提供帮助，为什么Postgres会对一个查询而不是另一个查询使用索引？ Running a VACUUM ANALYZE didn't help either. 运行VACUUM ANALYZE也无济于事。
Is there any way to speed up the queries? 有什么办法可以加快查询速度吗？ A query over the whole table would take about 20 - 30 minutes! 整个表的查询大约需要20-30分钟！ Partitioning didn't make much difference since queries span multiple partitions and it showed an improvement of only about a couple of minutes when the number of partitions spanned became larger.I have a postgres table with the following structure: 分区没有太大区别，因为查询跨越多个分区，并且当跨越的分区数量变大时，它仅显示了几分钟的改进。我有一个具有以下结构的postgres表：
+---------+-------------+-------------+----------+---------+---------+
| id | timestamp | numvalues | text1 | text2 | text3 |
+---------+-------------+-------------+----------+---------+---------+
|abcd12344| 4124135 |[1,2,53,1241]| apple | banana | papaya |
+---------+-------------+-------------+----------+---------+---------+
id - random alphanumeric value<br>
timestamp - epoch timestamp<br>
numvalues - array of integers<br>
text(n) - text values
The table has about 150 million rows. 该表约有1.5亿行。
I make an inner query to get the nth percentile of the data ordered on one of the values in the array. 我进行内部查询，以获取按数组值之一排序的数据的第n个百分点。 Then, I need to get the averages of several values from the array. 然后，我需要从数组中获取多个值的平均值。 The length of the array is around 31. 数组的长度约为31。
SELECT AVG(COALESCE(numvalues[2], 0))::NUMERIC(10,0), AVG(COALESCE(numvalues[3], 0))::NUMERIC(10,0)...AVG(COALESCE(numvalues[12], 0))::NUMERIC(10,0)
FROM (SELECT timestamp, numvalues, ntile(100)
OVER (ORDER BY numvalues[1]) pval FROM tablename WHERE timestamp >= somevalue and timestamp <= somevalue) innertable
WHERE pval >= x and pval <= y;
This returns about 7 million rows out of 150 million which is about 5% of the table. 这将返回1.5亿行中的约700万行，约占表的5％。 However, it does not use the index. 但是，它不使用索引。 Running an EXPLAIN ANALYZE shows that it uses a Seq Scan instead, even when enable_seqscan is set to off. 即使将enable_seqscan设置为off，运行EXPLAIN ANALYZE也会显示它使用Seq扫描代替。
However, a similar query : 但是，类似的查询：
SELECT text1, count(distinct(id))
FROM (SELECT timestamp, id, text1, numvalues, ntile(100)
OVER (ORDER BY numvalues[1]) pval FROM tablename WHERE timestamp >= somevalue and timestamp <= somevalue) innertable
WHERE pval >= x and pval <= y GROUP BY text1;
does use the index. 确实使用索引。
The index is on the timestamp column 索引在时间戳列上
The results of EXPLAIN ANALYZE: EXPLAIN ANALYZE的结果：
explain analyze select text1, count(distinct(id))
from (select timestamp, text1, numvalues, id, ntile(100) over (order by numvalues[1]) pval from table where timestamp >= 1431100800 and timestamp <= 1431108000 and numvalues[1] NOTNULL) innertable
where pval >= 90 and pval <= 90 group by text1;
QUERY PLAN
--------------------------------------------------------------------------------
GroupAggregate (cost=4554118.06..4554641.66 rows=1 width=28) (actual time=218641.221..219051.984 rows=20 loops=1)
-> Sort (cost=4554118.06..4554292.59 rows=69812 width=28) (actual time=218640.546..218728.294 rows=71441 loops=1)
Sort Key: innertable.text1
Sort Method: quicksort Memory: 8654kB
-> Subquery Scan on innertable (cost=4094722.75..4548501.27 rows=69812 width=28) (actual time=216502.946..218521.666 rows=71441 loops=1)
Filter: ((innertable.pval >= 90) AND (innertable.pval <= 90))
Rows Removed by Filter: 7072674
-> WindowAgg (cost=4094722.75..4339065.03 rows=13962416 width=118) (actual time=202276.333..211374.235 rows=7144115 loops=1)
-> Sort (cost=4094722.75..4129628.79 rows=13962416 width=118) (actual time=164912.487..190272.316 rows=7144115 loops=1)
Sort Key: (table.numvalues[9])
Sort Method: external merge Disk: 1387704kB
-> Index Scan using table_time_idx on table (cost=0.57..1578710.87 rows=13962416 width=118) (actual time=0.124..141014.505 rows=7144115 loops=1)
Index Cond: (("timestamp" >= 1431100800) AND ("timestamp" <= 1431108000))
Filter: (numvalues[1] IS NOT NULL)
Rows Removed by Filter: 7090075
Total runtime: 219340.709 ms
(16 rows)
explain analyze select avg(coalesce(numvalues[9], 0))::NUMERIC(10,0) mean9, avg(coalesce(numvalues[30],0))::NUMERIC(10,0), avg(coalesce(numvalues[8],0))::NUMERIC(10,0) - avg(coalesce(numvalues[30], 0))::NUMERIC(10,0) mean0, avg(coalesce(numvalues[10],0))::NUMERIC(10,0) - avg(coalesce(numvalues[8], 0))::NUMERIC(10,0) mean1, avg(coalesce(numvalues[14],0))::NUMERIC(10,0) - avg(coalesce(numvalues[10], 0))::NUMERIC(10,0) mean2, avg(coalesce(numvalues[13],0))::NUMERIC(10,0) - avg(coalesce(numvalues[14], 0))::NUMERIC(10,0) mean3, avg(coalesce(numvalues[9],0))::NUMERIC(10,0) - avg(coalesce(numvalues[13], 0))::NUMERIC(10,0) mean4
from (select timestamp, id, numvalues, ntile(100) over (order by numvalues[1] ) pval from table where timestamp >= 1431093600.00 and timestamp <= 1431100800.00 and numvalues[9] NOTNULL) innerTable
where pval >= 90.00 and pval <= 90.00 ;
QUERY PLAN
--------------------------------------------------------------------------------
Aggregate (cost=12662077.32..12662077.37 rows=1 width=82) (actual time=650343.769..650343.770 rows=1 loops=1)
-> Subquery Scan on innertable (cost=12634854.54..12661968.84 rows=3615 width=82) (actual time=647745.962..650232.725 rows=71441 loops=1)
Filter: (((innertable.pval)::numeric >= 90.00) AND ((innertable.pval)::numeric <= 90.00))
Rows Removed by Filter: 7072674
-> WindowAgg (cost=12634854.54..12647507.88 rows=723048 width=248) (actual time=632388.293..642338.237 rows=7144115 loops=1)
-> Sort (cost=12634854.54..12636662.16 rows=723048 width=248) (actual time=599893.771..617413.102 rows=7144115 loops=1)
Sort Key: (table.numvalues[9])
Sort Method: external merge Disk: 3214248kB
-> Seq Scan on req_p0swajch2t (cost=0.00..12480460.48 rows=723048 width=248) (actual time=0.041..575423.062 rows=7144115 loops=1)
Filter: ((numvalues[1] IS NOT NULL) AND (("timestamp")::numeric >= 1431100800.00) AND (("timestamp")::numeric <= 1431108000.00))
Rows Removed by Filter: 138191935
Total runtime: 650785.126 ms
(12 rows)
Can anyone help me out with why Postgres would use an index for one query and not the other? 谁能为我提供帮助，为什么Postgres会对一个查询而不是另一个查询使用索引？ Running a VACUUM ANALYZE didn't help either. 运行VACUUM ANALYZE也无济于事。
Is there any way to speed up the queries? 有什么办法可以加快查询速度吗？ A query over the whole table would take about 20 - 30 minutes! 整个表的查询大约需要20-30分钟！ Partitioning didn't make much difference since queries span multiple partitions and it showed an improvement of only about a couple of minutes when the number of partitions spanned became larger. 分区没有太大区别，因为查询跨越多个分区，并且当跨越的分区数量变大时，它仅显示了几分钟的改进。
1 个解决方案
解决方案1
3 已采纳 2015-06-08 10:37:06
I guess you can't reply in comments so I have to post an answer. 我想您无法在评论中回复，因此我必须发布答案。
Explain analyze showed, that your timestamp columns are compared with numeric values timestamp >= 1431093600.00 and timestamp <= 1431100800.00 and because of that they were cast to numeric: Explain analyze表明，您的时间戳列与数值timestamp >= 1431093600.00 and timestamp <= 1431100800.00 ，因此它们被timestamp >= 1431093600.00 and timestamp <= 1431100800.00为数字：
Filter: ((numvalues[1] IS NOT NULL) AND (("timestamp")::numeric >= 1431100800.00) AND (("timestamp")::numeric <= 1431108000.00))
Investigate why that happened and try to fix it. 调查发生这种情况的原因并尝试解决。
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