PostgreSQL 操作符与优化器详解 - 包含(选择性、JOIN方法、等效)等内容

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背景

PostgreSQL 支持自定义操作符,本质上是调用函数来实现的。

同时不同的操作符有幂等,反转,NOT等相关性,数据库的优化器可以利用这些关系,对QUERY进行REWRITE,产生更好的执行计划。

operator语法

语法如下:

例如创建一个求两个值的平均值的操作符:

首选要创建函数

postgres=# create function f_avg(numeric,numeric) returns numeric as $$  
postgres$#   select ($1+$2)/2;  
postgres$# $$ language sql strict;  
CREATE FUNCTION  

验证函数

postgres=# select f_avg(1,null);  
 f_avg   
-------  
        
(1 row)  
postgres=# select f_avg(1,2);  
       f_avg          
--------------------  
 1.5000000000000000  
(1 row)  

创建操作符,指定左右参数类型,调用的函数名,commutator是一个和优化器相关的选项,我后面会重点介绍:

postgres=# create operator ## (procedure=f_avg, leftarg=numeric, rightarg=numeric, commutator='##');  
CREATE OPERATOR  
postgres=# select 1 ## 2;  
      ?column?        
--------------------  
 1.5000000000000000  
(1 row)  

operator优化器开关

注意到在创建操作符的语法中有6个和优化器有关的关键字:

    [, COMMUTATOR = com_op ] [, NEGATOR = neg_op ]  
    [, RESTRICT = res_proc ] [, JOIN = join_proc ]  
    [, HASHES ] [, MERGES ]  

介绍如下:

假设x表示操作符左侧的参数,y表示操作符右侧的参数

1. commutator,指明x op1 y等效于y op2 x,即操作数调换,返回的值一样。例如2>1 和1<2结果是一致的。那么>就是<的commutator或者反之。又例如1+2和2+1是等价的,那么+就是+的commutator。commutator只需要在创建其中一个操作符时指定,创建另一个对应的操作符时可以不需要指定,PostgreSQL会自动建立这个关系。例如创建>操作符时指定了它的commutator是<,那么在创建<操作符时可以不需要指定>是它的commutator。

另外需要注意,有commutator操作符的操作符的左右两侧的参数类型必须一致,这样才能满足x op1 y等价于y op2 x。

优化器如何利用commutator呢?例如索引扫描,必须列在操作符的左侧才能使用索引。1 > tbl.c这个条件,如果>没有commutator的话,是不能使用索引的。

例子,以int4的>和<操作符为例,实验一下:

和<在PostgreSQL中是一对commutator

postgres=# select oprcom::regoper from pg_operator where oprname='>' and oprcode='int4gt'::regproc;  
    oprcom      
--------------  
 pg_catalog.<  
(1 row)  
postgres=# select oprcom::regoper from pg_operator where oprname='<' and oprcode='int4lt'::regproc;  
    oprcom      
--------------  
 pg_catalog.>  
(1 row)  

记录他们的oprcom对应的OID

postgres=# select * from pg_operator where oprname='>' and oprcode='int4gt'::regproc;  
 oprname | oprnamespace | oprowner | oprkind | oprcanmerge | oprcanhash | oprleft | oprright | oprresult | oprcom | oprnegate | oprc  
ode |   oprrest   |     oprjoin       
---------+--------------+----------+---------+-------------+------------+---------+----------+-----------+--------+-----------+-----  
----+-------------+-----------------  
 >       |           11 |       10 | b       | f           | f          |      23 |       23 |        16 |     97 |       523 | int4  
gt  | scalargtsel | scalargtjoinsel  
(1 row)  
postgres=# select * from pg_operator where oprname='<' and oprcode='int4lt'::regproc;  
 oprname | oprnamespace | oprowner | oprkind | oprcanmerge | oprcanhash | oprleft | oprright | oprresult | oprcom | oprnegate | oprc  
ode |   oprrest   |     oprjoin       
---------+--------------+----------+---------+-------------+------------+---------+----------+-----------+--------+-----------+-----  
----+-------------+-----------------  
 <       |           11 |       10 | b       | f           | f          |      23 |       23 |        16 |    521 |       525 | int4  
lt  | scalarltsel | scalarltjoinsel  
(1 row)  

接下来我要通过更新pg_operator解除他们的commutator关系,设置为0即可。

postgres=# update pg_operator set oprcom=0 where oprname='>' and oprcode='int4gt'::regproc;  
UPDATE 1  
postgres=# update pg_operator set oprcom=0 where oprname='<' and oprcode='int4lt'::regproc;  
UPDATE 1  

创建测试表,插入测试数据,创建索引:

postgres=# create table tbl(id int);  
CREATE TABLE  
postgres=# insert into tbl select generate_series(1,100000);  
INSERT 0 100000  
postgres=# create index idx_tbl_id on tbl(id);  
CREATE INDEX  

将列放在条件的左边可以走索引,但是放在右边不走索引。因为优化器不能决定>,<是否为commutator

postgres=# explain select * from tbl where id<10;  
                                QUERY PLAN                                   
---------------------------------------------------------------------------  
 Index Only Scan using idx_tbl_id on tbl  (cost=0.29..8.45 rows=9 width=4)  
   Index Cond: (id < 10)  
(2 rows)  
postgres=# explain select * from tbl where 10>id;  
                        QUERY PLAN                          
----------------------------------------------------------  
 Seq Scan on tbl  (cost=0.00..1361.00 rows=33333 width=4)  
   Filter: (10 > id)  
(2 rows)  

重新建立这两个 operator的commutator关系后,优化器会自动将10>id转换为id<10,并且走索引了:

postgres=# update pg_operator set oprcom=521 where oprname='<' and oprcode='int4lt'::regproc;  
UPDATE 1  
postgres=# update pg_operator set oprcom=97 where oprname='>' and oprcode='int4gt'::regproc;  
UPDATE 1  
postgres=# explain select * from tbl where 10>id;  
                                QUERY PLAN                                   
---------------------------------------------------------------------------  
 Index Only Scan using idx_tbl_id on tbl  (cost=0.29..8.45 rows=9 width=4)  
   Index Cond: (id < 10)  
(2 rows)  

2. negator,指x op1 y 等价于 not(y op2 x),或者x op1等价于not( y op2),或者op1 x 等价于not(op2 y),因此negator支持一元和二元操作符。

例子:

如果=和<>是一对negator操作符,NOT (x = y) 可以简化为 x <> y。

postgres=# explain select * from tbl where 10=id;  
                                QUERY PLAN                                   
---------------------------------------------------------------------------  
 Index Only Scan using idx_tbl_id on tbl  (cost=0.29..8.31 rows=1 width=4)  
   Index Cond: (id = 10)  
(2 rows)  
postgres=# explain select * from tbl where not(10<>id);  
                                QUERY PLAN                                   
---------------------------------------------------------------------------  
 Index Only Scan using idx_tbl_id on tbl  (cost=0.29..8.31 rows=1 width=4)  
   Index Cond: (id = 10)  
(2 rows)  

同样,操作符两侧参数x,y的类型必须一致。并且仅适用于返回布尔逻辑类型的操作符。

3. restrict,是用于评估选择性的函数,仅适用于二元操作符,例如where col>100,这个查询条件,如何评估选择性呢?是通过操作符的restrict来指定的,选择性乘以pg_class.reltuples就可以评估得到这个查询条件的行数。

选择性函数的代码在 src/backend/utils/adt/

包括

-rw-r--r--. 1 1107 1107  33191 Jun 10 03:29 array_selfuncs.c  
-rw-r--r--. 1 1107 1107   2316 Jun 10 03:29 geo_selfuncs.c  
-rw-r--r--. 1 1107 1107    720 Jun 10 03:29 network_selfuncs.c  
-rw-r--r--. 1 1107 1107  33895 Jun 10 03:29 rangetypes_selfuncs.c  
-rw-r--r--. 1 1107 1107 218809 Jun 10 03:29 selfuncs.c  

选择性函数,还需要依赖数据库的统计信息,从而计算选择性,常见的选择性计算函数有:

postgres=# select distinct oprrest from pg_operator order by 1;  
   oprrest      
--------------  
 -  
 eqsel  相等  
 neqsel  不相等  
 scalarltsel  小于等于  
 scalargtsel  大于等于  
 areasel  
 positionsel  
 contsel  
 iclikesel  
 icnlikesel  
 regexeqsel  
 likesel  
 icregexeqsel  
 regexnesel  
 nlikesel  
 icregexnesel  
 rangesel  
 networksel  
 tsmatchsel  
 arraycontsel  
(20 rows)  

当然,用户如果自定义数据类型的话,也可以自定义选择性函数,或者使用以上标准的选择性函数,只是可能需要实现一下类型转换。

源码中的介绍:

src/backend/utils/adt/selfuncs.c

/*----------  
 * Operator selectivity estimation functions are called to estimate the  
 * selectivity of WHERE clauses whose top-level operator is their operator.  
 * We divide the problem into two cases:  
 *              Restriction clause estimation: the clause involves vars of just  
 *                      one relation.  一种是符合WHERE条件的选择性(百分比)。  
 *              Join clause estimation: the clause involves vars of multiple rels.  
 * Join selectivity estimation is far more difficult and usually less accurate  
 * than restriction estimation.  -- JOIN的选择性评估通常没有WHERE条件的选择性准确。  
 *  
 * When dealing with the inner scan of a nestloop join, we consider the  
 * join's joinclauses as restriction clauses for the inner relation, and  
 * treat vars of the outer relation as parameters (a/k/a constants of unknown  
 * values).  So, restriction estimators need to be able to accept an argument  
 * telling which relation is to be treated as the variable.  
在使用nestloop JOIN时,一个表的字段将作为变量,另一个表的字段(及其统计信息)与操作符作为JOIN评估子句。  
 *  
 * The call convention for a restriction estimator (oprrest function) is  
 *  
 *              Selectivity oprrest (PlannerInfo *root,  
 *                                                       Oid operator,  
 *                                                       List *args,  
 *                                                       int varRelid);  
 * 评估选择性需要4个参数:  
 * root: general information about the query (rtable and RelOptInfo lists  
 * are particularly important for the estimator).   plannerinfo信息。  
 * operator: OID of the specific operator in question. 操作符的OID  
 * args: argument list from the operator clause.  操作符子句中的参数列表  
 * varRelid: if not zero, the relid (rtable index) of the relation to  
 * be treated as the variable relation.  May be zero if the args list  
 * is known to contain vars of only one relation.   表示where条件所包含的参数来自哪些relation。  
 *  
 * This is represented at the SQL level (in pg_proc) as  
 *  
 *              float8 oprrest (internal, oid, internal, int4);   在pg_proc数据字典中表示为oprrest指定的函数。  
 *  
 * The result is a selectivity, that is, a fraction (0 to 1) of the rows  
 * of the relation that are expected to produce a TRUE result for the  
 * given operator.  选择性函数的评估结果就是一个百分比。乘以pg_class.reltuples就可以得到记录数。  
 *  
 * The call convention for a join estimator (oprjoin function) is similar  
 * except that varRelid is not needed, and instead join information is  
 * supplied:  
 * JOIN选择性的计算函数与WHERE选择性的计算函数参数有轻微差别,么有varRelid, 增加了join信息的参数。  
 *              Selectivity oprjoin (PlannerInfo *root,  
 *                                                       Oid operator,  
 *                                                       List *args,  
 *                                                       JoinType jointype,  
 *                                                       SpecialJoinInfo *sjinfo);  
 *  
 *              float8 oprjoin (internal, oid, internal, int2, internal);  
 *  
 * (Before Postgres 8.4, join estimators had only the first four of these  
 * parameters.  That signature is still allowed, but deprecated.)  The  
 * relationship between jointype and sjinfo is explained in the comments for  
 * clause_selectivity() --- the short version is that jointype is usually  
 * best ignored in favor of examining sjinfo.  
 *  
 * Join selectivity for regular inner and outer joins is defined as the  
 * fraction (0 to 1) of the cross product of the relations that is expected  
 * to produce a TRUE result for the given operator.  For both semi and anti  (半连接与预连接)  
 * joins, however, the selectivity is defined as the fraction of the left-hand  
 * side relation's rows that are expected to have a match (ie, at least one  
 * row with a TRUE result) in the right-hand side.  
 *  
 * For both oprrest and oprjoin functions, the operator's input collation OID  
 * (if any) is passed using the standard fmgr mechanism, so that the estimator  
 * function can fetch it with PG_GET_COLLATION().  Note, however, that all  
 * statistics in pg_statistic are currently built using the database's default  
 * collation.  Thus, in most cases where we are looking at statistics, we  
 * should ignore the actual operator collation and use DEFAULT_COLLATION_OID.  
 * We expect that the error induced by doing this is usually not large enough  
 * to justify complicating matters.  
 *----------  

4. join,是joinsel即join的选择性计算函数。

对应pg_operator.oprjoin

postgres=# select distinct oprjoin from pg_operator order by 1;  
     oprjoin        
------------------  
 -  
 eqjoinsel  
 neqjoinsel  
 scalarltjoinsel  
 scalargtjoinsel  
 areajoinsel  
 positionjoinsel  
 contjoinsel  
 iclikejoinsel  
 icnlikejoinsel  
 regexeqjoinsel  
 likejoinsel  
 icregexeqjoinsel  
 regexnejoinsel  
 nlikejoinsel  
 icregexnejoinsel  
 networkjoinsel  
 tsmatchjoinsel  
 arraycontjoinsel  
(19 rows)  

5. hashes

6. merges

hashes和merges表示该操作符是否允许hash join和merge join, 只有返回布尔逻辑值的二元操作符满足这个要求。

我们在pg_operator这个catalog中也可以查看到对应的介绍:

Name Type References Description
oid oid - Row identifier (hidden attribute; must be explicitly selected)
oprname name - Name of the operator
oprnamespace oid pg_namespace.oid The OID of the namespace that contains this operator
oprowner oid pg_authid.oid Owner of the operator
oprkind char - b = infix (“between”), l = prefix (“left”), r = postfix (“right”) 指定操作符在什么位置,例如中间,左侧,右侧
oprcanmerge bool - This operator supports merge joins 此操作符是否支持merge join
oprcanhash bool - This operator supports hash joins 此操作符是否支持hash join
oprleft oid pg_type.oid Type of the left operand 操作符左侧的数据类型
oprright oid pg_type.oid Type of the right operand 操作符右侧的数据类型
oprresult oid pg_type.oid Type of the result 返回结果的数据类型
oprcom oid pg_operator.oid Commutator of this operator, if any  
oprnegate oid pg_operator.oid Negator of this operator, if any
oprcode regproc pg_proc.oid Function that implements this operator
oprrest regproc pg_proc.oid Restriction selectivity estimation function for this operator
oprjoin regproc pg_proc.oid Join selectivity estimation function for this operator

参考

1. http://www.postgresql.org/docs/9.4/static/sql-createoperator.html

2. http://www.postgresql.org/docs/9.4/static/xoper-optimization.html

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