Postgres分区修剪

Question

我在Postgres有一张大桌子。

表名是bigtable，列有：

integer |timestamp |xxx |xxx |...|xxx 
category_id|capture_time|col1|col2|...|colN 

我已经划分的capture_time列CATEGORY_ID和日期部分的模10表。

的分区表是这样的：

CREATE TABLE myschema.bigtable_d000h0(
    CHECK (category_id%10=0 AND capture_time >= DATE '2012-01-01' AND capture_time < DATE '2012-01-02') 
) INHERITS (myschema.bigtable); 

CREATE TABLE myschema.bigtable_d000h1(
    CHECK (category_id%10=1 AND capture_time >= DATE '2012-01-01' AND capture_time < DATE '2012-01-02') 
) INHERITS (myschema.bigtable); 

当我运行在where子句中使用CATEGORY_ID和capture_time查询，预期分区不修剪。

explain select * from bigtable where capture_time >= '2012-01-01' and capture_time < '2012-01-02' and category_id=100; 

"Result (cost=0.00..9476.87 rows=1933 width=216)" 
" -> Append (cost=0.00..9476.87 rows=1933 width=216)" 
"  -> Seq Scan on bigtable (cost=0.00..0.00 rows=1 width=210)" 
"    Filter: ((capture_time >= '2012-01-01 00:00:00'::timestamp without time zone) AND (capture_time < '2012-01-02 00:00:00'::timestamp without time zone) AND (category_id = 100))" 
"  -> Seq Scan on bigtable_d000h0 bigtable (cost=0.00..1921.63 rows=1923 width=216)" 
"    Filter: ((capture_time >= '2012-01-01 00:00:00'::timestamp without time zone) AND (capture_time < '2012-01-02 00:00:00'::timestamp without time zone) AND (category_id = 100))" 
"  -> Seq Scan on bigtable_d000h1 bigtable (cost=0.00..776.93 rows=1 width=218)" 
"    Filter: ((capture_time >= '2012-01-01 00:00:00'::timestamp without time zone) AND (capture_time < '2012-01-02 00:00:00'::timestamp without time zone) AND (category_id = 100))" 
"  -> Seq Scan on bigtable_d000h2 bigtable (cost=0.00..974.47 rows=1 width=216)" 
"    Filter: ((capture_time >= '2012-01-01 00:00:00'::timestamp without time zone) AND (capture_time < '2012-01-02 00:00:00'::timestamp without time zone) AND (category_id = 100))" 
"  -> Seq Scan on bigtable_d000h3 bigtable (cost=0.00..1351.92 rows=1 width=214)" 
"    Filter: ((capture_time >= '2012-01-01 00:00:00'::timestamp without time zone) AND (capture_time < '2012-01-02 00:00:00'::timestamp without time zone) AND (category_id = 100))" 
"  -> Seq Scan on bigtable_d000h4 bigtable (cost=0.00..577.04 rows=1 width=217)" 
"    Filter: ((capture_time >= '2012-01-01 00:00:00'::timestamp without time zone) AND (capture_time < '2012-01-02 00:00:00'::timestamp without time zone) AND (category_id = 100))" 
"  -> Seq Scan on bigtable_d000h5 bigtable (cost=0.00..360.67 rows=1 width=219)" 
"    Filter: ((capture_time >= '2012-01-01 00:00:00'::timestamp without time zone) AND (capture_time < '2012-01-02 00:00:00'::timestamp without time zone) AND (category_id = 100))" 
"  -> Seq Scan on bigtable_d000h6 bigtable (cost=0.00..1778.18 rows=1 width=214)" 
"    Filter: ((capture_time >= '2012-01-01 00:00:00'::timestamp without time zone) AND (capture_time < '2012-01-02 00:00:00'::timestamp without time zone) AND (category_id = 100))" 
"  -> Seq Scan on bigtable_d000h7 bigtable (cost=0.00..315.82 rows=1 width=216)" 
"    Filter: ((capture_time >= '2012-01-01 00:00:00'::timestamp without time zone) AND (capture_time < '2012-01-02 00:00:00'::timestamp without time zone) AND (category_id = 100))" 
"  -> Seq Scan on bigtable_d000h8 bigtable (cost=0.00..372.06 rows=1 width=219)" 
"    Filter: ((capture_time >= '2012-01-01 00:00:00'::timestamp without time zone) AND (capture_time < '2012-01-02 00:00:00'::timestamp without time zone) AND (category_id = 100))" 
"  -> Seq Scan on bigtable_d000h9 bigtable (cost=0.00..1048.16 rows=1 width=215)" 
"    Filter: ((capture_time >= '2012-01-01 00:00:00'::timestamp without time zone) AND (capture_time < '2012-01-02 00:00:00'::timestamp without time zone) AND (category_id = 100))" 

但是，如果我在where子句中添加的确切模标准（category_id%10=0），它完美

explain select * from bigtable where capture_time >= '2012-01-01' and capture_time < '2012-01-02' and category_id=100 and category_id%10=0; 

"Result (cost=0.00..2154.09 rows=11 width=215)" 
" -> Append (cost=0.00..2154.09 rows=11 width=215)" 
"  -> Seq Scan on bigtable (cost=0.00..0.00 rows=1 width=210)" 
"    Filter: ((capture_time >= '2012-01-01 00:00:00'::timestamp without time zone) AND (capture_time < '2012-01-02 00:00:00'::timestamp without time zone) AND (category_id = 100) AND ((category_id % 10) = 0))" 
"  -> Seq Scan on bigtable_d000h0 bigtable (cost=0.00..2154.09 rows=10 width=216)" 
"    Filter: ((capture_time >= '2012-01-01 00:00:00'::timestamp without time zone) AND (capture_time < '2012-01-02 00:00:00'::timestamp without time zone) AND (category_id = 100) AND ((category_id % 10) = 0))" 

有没有什么办法让分区修剪工作正常，而无需添加模每个查询中的条件？

Answer 1

对于任何人谁具有相同的问题： 我得出的结论是，从最简单的方法是改变的查询，包括您正在使用哪个版本的模条件category_id%10=0

Answer 2

事情是：用于排除约束PostgreSQL will create an implicit index。在你的情况下，这个索引将是一个部分，'因为你在列上使用expresion，而不仅仅是它的价值。而且它在documentation规定（寻找11-2例）：

PostgreSQL没有复杂的理论校能够识别那些形式不同但数学上等价的表达。 （不仅是这样的一般定理证明者极难创建，它可能太慢而不能真正用到）。系统可以识别简单的不等式含义，例如“x < 1”意味着“x < 2”; 否则谓词条件必须与查询的WHERE条件的一部分完全匹配，否则索引将不会被识别为可用。匹配发生在查询计划时间，而不是在运行时。

因此，您的结果 - 您应该有创建CHECK约束时所使用的完全相同的表达式。

对于基于散列的分区我更喜欢2点的方法：

• 添加可（在壳体10）取一组有限值中的一个字段，最好在由设计存在这样一个;
• 指定哈希范围指定时间戳以同样的方式范围：MINVALUE < = CATEGORY_ID < MAXVALUE

此外，还可以创建一个2级分区：第一个层次，你

• 根据category_id HASH创建10个分区;
• 在第二级上，您可以根据日期范围创建必要数量的分区。

尽管我总是试图只使用1列进行分区，但更容易管理。