本文分享自华为云社区《【SQL优化】为什么有时候无法走执行性能更优的hashjoin》，作者： leapdb。
1. hash join通常优于nestloop 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：1. 在session级关闭nestloop方式，set enable_nestloop to off;
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)
不同数据类型计算hash函数不同，互不兼容的数据类型无法进行hash比较。
2. 为什么hashjoin秒级，nestloop需要两个小时nestloop复杂度：131w * 10w = 1310亿
hashjoin复杂度：131w
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看似类型相近，但由于两端的精度，格式，有无时区等不一样，无法认为直接相等。
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TD兼容模式下为date类型；
current_date 在Oracle兼容模式下为timestamp类型；标签：code,join,timestamp,SQL,date,hash,day From： https://www.cnblogs.com/huaweiyun/p/17761544.html