表扫描方式
表扫描方式主要包含顺序扫描、索引扫描以及Tid扫描等方式,不同的扫描方式
- Seq scan,顺序扫描物理数据页
- Index scan,先通过索引值获得物理数据的位置,再到物理页读取
QUERY PLAN
--------------------------------------------------------------------
Index Scan using t1_a1_key on t1 (cost=0.28..8.29 rows=1 width=8)
Index Cond: (a1 = 10)
- Tid scan,通过page号和item号直接定位到物理数据
postgres=> explain select * from t1 where ctid='(1,10)';
QUERY PLAN
--------------------------------------------------
Tid Scan on t1 (cost=0.00..4.01 rows=1 width=8)
TID Cond: (ctid = '(1,10)'::tid)
选择度计算
- 全表扫描选择度计算
全表扫描时每条记录都会返回,所以选择度为1,所以rows=10000
EXPLAIN SELECT * FROM tenk1;
QUERY PLAN
-------------------------------------------------------------
Seq Scan on tenk1 (cost=0.00..458.00 rows=10000 width=244)
relpages | reltuples
358 | 10000
- 整型大于或者小于选择度计算
- 字符串等值选择度计算
EXPLAIN SELECT * FROM tenk1 WHERE stringu1 = 'CRAAAA';
QUERY PLAN
----------------------------------------------------------
Seq Scan on tenk1 (cost=0.00..483.00 rows=30 width=244)
Filter: (stringu1 = 'CRAAAA'::name)
SELECT null_frac, n_distinct, most_common_vals, most_common_freqs FROM pg_stats
WHERE tablename='tenk1' AND attname='stringu1';
null_frac | 0
n_distinct | 676
most_common_vals|{EJAAAA,BBAAAA,CRAAAA,FCAAAA,FEAAAA,GSAAAA,JOAAAA,MCAAAA,NAAAAA,WGAAAA}
most_common_freqs | {0.00333333,0.003,0.003,0.003,0.003,0.003,0.003,0.003,0.003,0.003}
selectivity = mcf[3]
= 0.003
rows = 10000 * 0.003
= 30
备注:如果值不在most_common_vals里面,计算公式为:
selectivity = (1 - sum(mvf))/(num_distinct - num_mcv)
- cost计算
代价模型:总代价=CPU代价+IO代价+启动代价
-----------------------------------------------------
Seq Scan on t1 (cost=0.00..16.90 rows=942 width=8)
Filter: (a1 > 10)
(2 rows)
其中:
postgres=> select relpages, reltuples from pg_class where relname = 't1';
relpages | reltuples
----------+-----------
5 | 952
(1 row)
cpu_operator_cost=0.0025
cpu_tuple_cost=0.01
seq_page_cost=1
random_page_cost=4
总cost = cpu_tuple_cost * 952 + seq_page_cost * 5 + cpu_operator_cost * 952 = 16.90 其他扫描方式cost计算可以参考如下函数:
表组合方式
- Nest Loop
SELECT * FROM t1 L, t2 R WHERE L.id=R.id
M = 20000 pages in L, pL = 40 rows per page, N = 400 pages in R, pR = 20 rows per page.
select relpages, reltuples from pg_class where relname=‘t1’
L和R进行join
for l in L do
for r in R do
if rid == lid then ret += (r, s)
对于外表L每一个元组扫描内表R所有的元组 总IO代价: M + (pL * M) * N = 20000 + (4020000)400
\= 320020000
- MergeJoin
主要分为3步:
(1) Sort L on lid 代价MlogM
(3) Merge the sorted L and R on lid and rid 代价M+N
- HashJoin
使用HashJoin的前提是其中假设一个表可以完全放在内存中,实际过程中可能统计信息有偏差,优化器认为一个表可以放到内存中,事实上数据在内存中放不下,需要使用临时文件,这样会降低性能。
表的组合顺序
不同的组合顺序将会产生不同的代价,想要获得最佳的组合顺序,如果枚举所有组合顺序,那么将会有N!的排列组合,计算量对于优化器来说难以承受。PG优化器使用两种算法计算更优的组合顺序,动态规划和遗传算法。对于连接比较少的情况使用动态规划,否则使用遗传算法。
- 动态规划求解过程
PG优化器主要考虑将执行计划树生成以下三种形式: