I'm confused over two question involving PostgreSQL index scans. I'm using
Linux Mandrake 7.2 on Intel, and PostgreSQL 7.0.3. If someone can explain
what's going on, I'd greatly appreciate it.
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1) When I create a empty table, and then immediate create an index on a
column, I can get /index scans/ when searching on that column. But when I
then run VACUUM, the same search becomes a /sequential scan/. Shouldn't it
still be an index scan? What's going on here?
test1=> create table t1 (a varchar(64), b int);
CREATE
test1=> create index t1_a_ndx on t1 (a);
CREATE
test1=> explain select * from t1 where a='asd';
NOTICE: QUERY PLAN:
Index Scan using t1_a_ndx on t1 (cost=0.00..8.14 rows=10 width=16)
EXPLAIN
test1=> vacuum;
NOTICE: Skipping "pg_type" --- only table owner can VACUUM it (a bunch of
these)
VACUUM
test1=> explain select * from t1 where a='asd';
NOTICE: QUERY PLAN:
Seq Scan on t1 (cost=0.00..0.00 rows=1 width=16)
EXPLAIN
test1=>
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2) If I already have some data in a table and I create an index on a
column, why doesn't subsequent searches then change from sequential scans
to index scans?
test1=> create table t2 (a varchar(64), b int);
CREATE
test1=> insert into t2 values ('a', 1);
INSERT 41255 1
test1=> insert into t2 values ('b', 2);
INSERT 41256 1
test1=> insert into t2 values ('c', 3);
INSERT 41257 1
test1=> explain select * from t2 where a='a';
NOTICE: QUERY PLAN:
Seq Scan on t2 (cost=0.00..22.50 rows=10 width=16)
EXPLAIN
test1=> create index t2_a_ndx on t2 (a);
CREATE
test1=> explain select * from t2 where a='a';
NOTICE: QUERY PLAN:
Seq Scan on t2 (cost=0.00..1.04 rows=1 width=16)
EXPLAIN
test1=>
