数据量很小的时候，我们可以看到，seq scan 比 index scan 更加有效。那是因为 index scan 至少要发生两次I/O，一次是 读取索引块， 一次是读取数据块。当index 很大的时候，情况可能会更加复杂。
postgres=# select a.relpages,a.reltuples,a.relfilenode,a.reltype,b.typname from pg_class a,pg_type b where a.relname like 'gaotab%' and a.reltype=b.oid;
relpages | reltuples | relfilenode | reltype | typname ----------+-----------+-------------+---------+---------
1 | 100 | 16396 | 16386 | gaotab
数据量为 100条记录。
预估成本：
set session enable_seqscan=false;
SET
postgres=# explain select name from gaotab where id=50;
QUERY PLAN ---------------------------------------------------------------------
Index Scan using idx_id on gaotab (cost=0.00..8.27 rows=1 width=5)
Index Cond: (id = 50) (2 rows)
true;
QUERY PLAN ------------------------------------------------------
Seq Scan on gaotab (cost=0.00..2.25 rows=1 width= Filter: (id = 2 rows)
实际执行：
postgres=# explain analyze QUERY PLAN -------------------------------------------------------------------------------- -------------------------------
5) (actual tim e=0.112..0.113 rows=1 loops=1)
50) Total runtime: 0.133 ms (3 rows)
QUERY PLAN -------------------------------------------------------------------------------- ----------------
Seq Scan on gaotab (cost=2.25 rows=5) (actual time=0.014..0.018 rows= Filter: (id = 50) Rows Removed by Filter: 99 Total runtime: 0.034 ms (4 rows)
等到数据量大的时候，就是截然不同了。
数据为1000条记录时，通过查询可以看到，已经跨越了7个page：
postgres=# analyze;
ANALYZE
postgres=# 7 | 1000 | 16386 | gaotab (1 row)
postgres=#
再次预估成本，此时seq scan 已经开始变得不划算了：
postgres=# explain select name from gaotab where id=QUERY PLAN ---------------------------------------------------------------------
usingidx_id on gaotab (cost=8.276)
Index Cond: (id 2 rows)
=# set session enable_seqscan= session enable_indexscan=Bitmap Heap Scan on gaotab (cost4.26 Recheck Cond: (id 50)
-> Bitmap Index Scan on idx_id (cost=0)
4 rows)
session enable_bitmapscan=-------------------------------------------------------
Seq Scan on gaotab (cost19.50=#
实际执行
QUERY PLAN ------------------------------------------------------------------------------------------------------------
原文链接：https://www.f2er.com/postgresql/195741.html10000000000.00..10000000008.27 rows=6) (actual time=0.020. .0.022 rows=1) Index Cond: (id = 0.051 ms (3 rows)
set session enable_indexscan=set session enable_bitmapscan=QUERY PLAN -------------------------------------------------------------------------------------------------
19.50 rows=0.015..0.095 rows=999 Total runtime: 0.109 ms (postgres=#
