postgres执行计划 seqscan,indexscan,bitmapscan实验性分析

执行以下代码

create table t1(c1 int4);
insert into t1 select generate_series(1,10);
explain analyze select * from t1 where  c1 =4

显示为

"Seq Scan on t1  (cost=0.00..40.00 rows=12 width=4) (actual time=0.007..0.007 rows=1 loops=1)"
"  Filter: (c1 = 4)"
"Total runtime: 0.022 ms"

这本没有什么问题，现在加上索引

create index t1_index on t1 (c1);
explain analyze select * from t1 where  c1 =4

显示依然为

"Seq Scan on t1  (cost=0.00..1.13 rows=1 width=4) (actual time=0.013..0.015 rows=1 loops=1)"
"  Filter: (c1 = 4)"
"Total runtime: 0.050 ms"

执行计划并未改变依然为全表扫描加filter！

那索引目的何在？使用如下代码强制走索引测试

SET enable_seqscan TO off;
SET enable_indexscan TO on;

explain ana