lightdb22.4-新增优化器提示cardinality 和ordered_predicates
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lightdb 22.4 新增优化器提示cardinality 和ordered_predicates
lightdb 在22.4 新增了如下两个优化器提示：
- cardinality
- ordered hint
下面对这两个hint 进行介绍
1. cardinality hint
cardinality hint 用于强制设置表或多表查询结果的预估行数，从而影响优化器选择执行路径，hint的效果体现在对explain 中rows值的印象。
具体请看如下案例：
1.0 前置准备
lightdb@postgres=# create table t1 (key1 int , key2 int);
CREATE TABLE
lightdb@postgres=# create table t2 (key1 int , key2 int);
CREATE TABLE
1.1 影响join算法选择
通过使用cardinality设置t1表预估行数为10，导致走了hashjoin：
lightdb@postgres=# explain select * from t1, t2 where t1.key1 = t2.key1;
QUERY PLAN
------------------------------------------------------------------
Merge Join (cost=317.01..711.38 rows=25538 width=16)
Merge Cond: (t1.key1 = t2.key1)
-> Sort (cost=158.51..164.16 rows=2260 width=8)
Sort Key: t1.key1
-> Seq Scan on t1 (cost=0.00..32.60 rows=2260 width=8)
-> Sort (cost=158.51..164.16 rows=2260 width=8)
Sort Key: t2.key1
-> Seq Scan on t2 (cost=0.00..32.60 rows=2260 width=8)
(8 rows)
lightdb@postgres=# explain select /*+cardinality(t1 10)*/* from t1, t2 where t1.key1 = t2.key1;
QUERY PLAN
------------------------------------------------------------------------
Hash Join (cost=32.73..74.93 rows=113 width=16)
Hash Cond: (t2.key1 = t1.key1)
-> Seq Scan on t2 @"lt#0" (cost=0.00..32.60 rows=2260 width=8)
-> Hash (cost=32.60..32.60 rows=10 width=8)
-> Seq Scan on t1 @"lt#0" (cost=0.00..32.60 rows=10 width=8)
(5 rows)
lightdb@postgres=#
1.2 影响内外表选择
对应hashjoin，可以通过把rows 设小，可以强制把设置的表作为内表构建hash表。
lightdb@postgres=# explain select /*+cardinality(t1 20) cardinality(t2 20)*/* from t1, t2 where t1.key1 = t2.key1;
QUERY PLAN
------------------------------------------------------------------------
Hash Join (cost=32.85..65.57 rows=2 width=16)
Hash Cond: (t1.key1 = t2.key1)
-> Seq Scan on t1 @"lt#0" (cost=0.00..32.60 rows=20 width=8)
-> Hash (cost=32.60..32.60 rows=20 width=8)
-> Seq Scan on t2 @"lt#0" (cost=0.00..32.60 rows=20 width=8)
(5 rows)
lightdb@postgres=# explain select /*+cardinality(t1 10) cardinality(t2 20)*/* from t1, t2 where t1.key1 = t2.key1;
QUERY PLAN
------------------------------------------------------------------------
Hash Join (cost=32.73..65.41 rows=1 width=16)
Hash Cond: (t2.key1 = t1.key1)
-> Seq Scan on t2 @"lt#0" (cost=0.00..32.60 rows=20 width=8)
-> Hash (cost=32.60..32.60 rows=10 width=8)
-> Seq Scan on t1 @"lt#0" (cost=0.00..32.60 rows=10 width=8)
(5 rows)
lightdb@postgres=# explain select /*+cardinality(t1 20) cardinality(t2 10)*/* from t1, t2 where t1.key1 = t2.key1;
QUERY PLAN
------------------------------------------------------------------------
Hash Join (cost=32.73..65.41 rows=1 width=16)
Hash Cond: (t1.key1 = t2.key1)
-> Seq Scan on t1 @"lt#0" (cost=0.00..32.60 rows=20 width=8)
-> Hash (cost=32.60..32.60 rows=10 width=8)
-> Seq Scan on t2 @"lt#0" (cost=0.00..32.60 rows=10 width=8)
(5 rows)
lightdb@postgres=#
1.3 影响子查询
lightdb@postgres=# explain select t1.key1 from t1, (select distinct * from t2 ) x where t1.key1 = x.key1 and x.key2>10;
QUERY PLAN
-----------------------------------------------------------------------------
Hash Join (cost=49.23..174.72 rows=2509 width=4)
Hash Cond: (t1.key1 = x.key1)
-> Seq Scan on t1 (cost=0.00..32.60 rows=2260 width=4)
-> Hash (cost=46.45..46.45 rows=222 width=4)
-> Subquery Scan on x (cost=42.02..46.45 rows=222 width=4)
-> HashAggregate (cost=42.02..44.23 rows=222 width=8)
Group Key: t2.key1, t2.key2
-> Seq Scan on t2 (cost=0.00..38.25 rows=753 width=8)
Filter: (key2 > 10)
(9 rows)
lightdb@postgres=# explain select /*+cardinality(x 100)*/t1.key1 from t1, (select distinct * from t2 ) x where t1.key1 = x.key1 and x.key2>10;
QUERY PLAN
-----------------------------------------------------------------------------
--------
Hash Join (cost=47.70..125.50 rows=1130 width=4)
Hash Cond: (t1.key1 = x.key1)
-> Seq Scan on t1 @"lt#0" (cost=0.00..32.60 rows=2260 width=4)
-> Hash (cost=46.45..46.45 rows=100 width=4)
-> Subquery Scan on x @"lt#0" (cost=42.02..46.45 rows=100 width=4)
-> HashAggregate (cost=42.02..44.23 rows=222 width=8)
Group Key: t2.key1, t2.key2
-> Seq Scan on t2 @"lt#1" (cost=0.00..38.25 rows=753 w
idth=8)
Filter: (key2 > 10)
(9 rows)
lightdb@postgres=#
1.4 影响多表
lightdb@postgres=# explain select * from t1 ,t2 ,t1 a where t1.key1=t2.key1 and t2.key1=a.key1;
QUERY PLAN
-----------------------------------------------------------------------------
-
Merge Join (cost=475.52..5209.87 rows=288579 width=24)
Merge Cond: (a.key1 = t1.key1)
-> Sort (cost=158.51..164.16 rows=2260 width=8)
Sort Key: a.key1
-> Seq Scan on t1 a (cost=0.00..32.60 rows=2260 width=8)
-> Materialize (cost=317.01..775.23 rows=25538 width=16)
-> Merge Join (cost=317.01..711.38 rows=25538 width=16)
Merge Cond: (t1.key1 = t2.key1)
-> Sort (cost=158.51..164.16 rows=2260 width=8)
Sort Key: t1.key1
-> Seq Scan on t1 (cost=0.00..32.60 rows=2260 width=8)
-> Sort (cost=158.51..164.16 rows=2260 width=8)
Sort Key: t2.key1
-> Seq Scan on t2 (cost=0.00..32.60 rows=2260 width=8)
(14 rows)
lightdb@postgres=# explain select /*+cardinality(t1 t2 100)*/* from t1 ,t2 ,t1 a where t1.key1=t2.key1 and t2.key1=a.key1;
QUERY PLAN
-----------------------------------------------------------------------------
---------
Hash Join (cost=712.63..790.43 rows=1130 width=24)
Hash Cond: (a.key1 = t1.key1)
-> Seq Scan on t1 a @"lt#0" (cost=0.00..32.60 rows=2260 width=8)
-> Hash (cost=711.38..711.38 rows=100 width=16)
-> Merge Join (cost=317.01..711.38 rows=100 width=16)
Merge Cond: (t1.key1 = t2.key1)
-> Sort (cost=158.51..164.16 rows=2260 width=8)
Sort Key: t1.key1
-> Seq Scan on t1 @"lt#0" (cost=0.00..32.60 rows=2260
width=8)
-> Sort (cost=158.51..164.16 rows=2260 width=8)
Sort Key: t2.key1
-> Seq Scan on t2 @"lt#0" (cost=0.00..32.60 rows=2260
width=8)
(12 rows)
lightdb@postgres=#
note
cardinality hint是rows hint 的别名，在22.4 我们对rows hint 进行了扩展，支持对单表进行设置。
2. ordered_predicates hint
ordered_predicates 用于强制优化器保留约束的顺序，对索引条件无效, 不考虑连接条件。目前具有如下限制：
- 对于涉及等价推导的约束不起效，包括实际参与推导，和推导参数的约束。
具体案例如下：
2.0 前置准备
chuhx@postgres=# create table test1 (key1 int, key2 int, key3 int);
CREATE TABLE
chuhx@postgres=# create table test2 (key1 int, key2 int, key3 int);
CREATE TABLE
chuhx@postgres=#
2.1 对于where 或on或having后约束 不组合
where
lightdb@postgres=# EXPLAIN (COSTS false) select * from test1 where mod(key2, 10) = 2 and key1 = 1;
QUERY PLAN
------------------------------------------------
Seq Scan on test1
Filter: ((key1 = 1) AND (mod(key2, 10) = 2))
(2 rows)
lightdb@postgres=# EXPLAIN (COSTS false) select/*+ordered_predicates*/ * from test1 where mod(key2, 10) = 2 and key1 = 1;
QUERY PLAN
------------------------------------------------
Seq Scan on test1 @"lt#0"
Filter: ((mod(key2, 10) = 2) AND (key1 = 1))
(2 rows)
lightdb@postgres=# EXPLAIN (COSTS false) select * from test1 where mod(key2, 10) = 2 and key1 > 1;
QUERY PLAN
------------------------------------------------
Seq Scan on test1
Filter: ((key1 > 1) AND (mod(key2, 10) = 2))
(2 rows)
lightdb@postgres=# EXPLAIN (COSTS false) select/*+ordered_predicates*/ * from test1 where mod(key2, 10) = 2 and key1 > 1;
QUERY PLAN
------------------------------------------------
Seq Scan on test1 @"lt#0"
Filter: ((mod(key2, 10) = 2) AND (key1 > 1))
(2 rows)
on
lightdb@postgres=# EXPLAIN (COSTS false) select * from test1 a join test2 b on a.key1=b.key1 and mod(a.key2, 10) = 2 and a.key3 = 10;
QUERY PLAN
-------------------------------------------------------------
Hash Join
Hash Cond: (b.key1 = a.key1)
-> Seq Scan on test2 b
-> Hash
-> Seq Scan on test1 a
Filter: ((key3 = 10) AND (mod(key2, 10) = 2))
(6 rows)
lightdb@postgres=# EXPLAIN (COSTS false) select/*+ordered_predicates*/ * from test1 a join test2 b on a.key1=b.key1 and mod(a.key2, 10) = 2 and a.key3 = 10;
QUERY PLAN
-------------------------------------------------------------
Hash Join
Hash Cond: (b.key1 = a.key1)
-> Seq Scan on test2 b @"lt#0"
-> Hash
-> Seq Scan on test1 a @"lt#0"
Filter: ((mod(key2, 10) = 2) AND (key3 = 10))
(6 rows)
having
lightdb@postgres=# EXPLAIN (COSTS false) select key1 from test1 group by key1, key2 having mod(key2, 10) = 2 and key1 = 1;
QUERY PLAN
------------------------------------------------------------
Group
Group Key: key1, key2
-> Sort
Sort Key: key2
-> Seq Scan on test1
Filter: ((key1 = 1) AND (mod(key2, 10) = 2))
(6 rows)
lightdb@postgres=# EXPLAIN (COSTS false) select/*+ordered_predicates*/ key1 from test1 group by key1, key2 having mod(key2, 10) = 2 and key1 = 1;
QUERY PLAN
------------------------------------------------------------
Group
Group Key: key1, key2
-> Sort
Sort Key: key2
-> Seq Scan on test1 @"lt#0"
Filter: ((mod(key2, 10) = 2) AND (key1 = 1))
(6 rows)
2.2 on 和where 结合（无等价推导， having 类同where，不再举例）
lightdb@postgres=# EXPLAIN (COSTS false) select * from test1 a join test2 b on a.key1=b.key1 and mod(a.key2, 10) = 2 where mod(b.key2, 10) = 2 and a.key3 = 10;
QUERY PLAN
-------------------------------------------------------
Nested Loop
Join Filter: (a.key1 = b.key1)
-> Seq Scan on test1 a
Filter: ((key3 = 10) AND (mod(key2, 10) = 2))
-> Seq Scan on test2 b
Filter: (mod(key2, 10) = 2)
(6 rows)
lightdb@postgres=# EXPLAIN (COSTS false) select/*+ordered_predicates*/ * from test1 a join test2 b on a.key1=b.key1 and mod(a.key2, 10) = 2 where mod(b.key2, 10) = 2 and a.key3 = 10;
QUERY PLAN
-------------------------------------------------------
Nested Loop
Join Filter: (a.key1 = b.key1)
-> Seq Scan on test1 a @"lt#0"
Filter: ((mod(key2, 10) = 2) AND (key3 = 10))
-> Seq Scan on test2 b @"lt#0"
Filter: (mod(key2, 10) = 2)
(6 rows)
2.3 不起效情况
有等价类推导a.key2=b.key2 && b.key2 = 1 导致 b 表上的 ((key2 = 1) AND (mod(key2, 10) = 2)) 不能保持sql 中定义顺序。
lightdb@postgres=# EXPLAIN (COSTS false) select * from test1 a left join test2 b on a.key1=b.key1 and a.key2=b.key2 where mod(b.key2, 10) = 2 and b.key2 = 1;
e QUERY PLAN
y2 where mod(b.key2, 10) = 2 and b.key2 = 1;
------------------------------------------------------
Nested Loop
Join Filter: (a.key1 = b.key1)
-> Seq Scan on test2 b
Filter: ((key2 = 1) AND (mod(key2, 10) = 2))
-> Seq Scan on test1 a
Filter: (key2 = 1)
(6 rows)
lightdb@postgres=# EXPLAIN (COSTS false) select/*+ordered_predicates*/ * from test1 a left j以上是关于lightdb22.4-新增优化器提示cardinality 和ordered_predicates的主要内容，如果未能解决你的问题，请参考以下文章