如何强制SQL走性能更优的hash join

通常nestloop join的复杂度是O(N方)，hash join时间复杂度是O(N)，所以我们一般倾向于使用hash join。

 

在SQL脚本调优过程中通常有两种方式，强制走hash join方式：

2. 在SQL中通过 /*+ hashjoin(a b) */ 方式，让a和b表走hash join;

 

CREATE DATABASE test_td WITH DBCOMPATIBILITY='td'; create table dim_day(day_code char(8)); create table dwr_rpo as select current_date - 1 as day_code; --返回了date类型 test_td=# \d+ dwr_rpo Table "public.dwr_rpo" Column | Type | Modifiers | Storage | Stats target | Description ----------+------+-----------+---------+--------------+------------- day_code | date | | plain | | Has OIDs: no Distribute By: ROUND ROBIN Location Nodes: ALL DATANODES Options: orientation=row, compression=no explain select * from dwr_rpo a left join dim_day c on c.day_code = a.day_code; id | operation | E-rows | E-distinct | E-memory | E-width | E-costs ---+----------------------------------------------+---------+------------+----------+---------+-------------- 1 | -> Streaming (type: GATHER) | 1310148 | | | 1694 | 279235196.70 2 | -> Nested Loop Left Join (3, 4) | 1310148 | | 1MB | 1694 | 279229682.93 3 | -> Seq Scan on dwr_rpo a | 1310148 | | 1MB | 1676 | 46589.16 4 | -> Materialize | 109575 | | 16MB | 22 | 3747.76 5 | -> Streaming(type: BROADCAST) | 109575 | | 2MB | 22 | 3565.14 6 | -> Seq Scan on dim_day c | 36525 | | 1MB | 22 | 272.75 Predicate Information (identified by plan id) ----------------------------------------------------------------------------- 2 --Nested Loop Left Join (3, 4) Join Filter: ((c.day_code)::timestamp without time zone = a.day_code)

可是，以上SQL无论用哪种方式都走不上hash join。我们需要看一下，join两端的数据类型是否支持hash比较。

 

 

1. 为什么有时候无法走执行性能更优的hashjoin

 

2. 为什么hashjoin秒级，nestloop需要两个小时

所以两种方式性能差距很大。

 

3. 为什么有类型转换，还不能hash join

4. 都哪些数据类型间的join不支持hash?

select oprname,oprkind,oprcanhash, (select typname from pg_type where oid=oprleft) oprleft, (select typname from pg_type where oid=oprright) oprright from pg_operator where oprname='=' and oprcanhash='f'; oprname | oprkind | oprcanhash | oprleft | oprright ---------+---------+------------+---------------+--------------- = | b | f | xid | int8 = | b | f | xid32 | int4 = | b | f | tid | tid = | b | f | box | box = | b | f | path | path = | b | f | tinterval | tinterval = | b | f | money | money = | b | f | circle | circle = | b | f | lseg | lseg = | b | f | line | line = | b | f | bit | bit = | b | f | varbit | varbit = | b | f | date | timestamp = | b | f | date | timestamptz = | b | f | timestamp | date = | b | f | timestamptz | date = | b | f | timestamp | timestamptz = | b | f | timestamptz | timestamp = | b | f | tsvector | tsvector = | b | f | tsquery | tsquery = | b | f | record | record = | b | f | hll | hll = | b | f | hll_hashval | hll_hashval = | b | f | roaringbitmap | roaringbitmap (24 rows)

 

主要是timestamp, timestamptz, date间互相join是无法走hash。其它数据类型不常见。

 

开发建议：join两端的数据类型尽量一致或互相兼容。

 

5. 为什么 oracle 兼容模式没有问题，td兼容模式有问题？

current_date 在Oracle兼容模式下为timestamp类型；