Re: Unused index influencing sequential scan plan

On 18 October 2012 17:11, Thom Brown <thom(at)linux(dot)com> wrote:
> Hi all,
>
> I've created a test table containing 21 million random dates and
> times, but I get wildly different results when I introduce a
> functional index then ANALYSE again, even though it doesn't use the
> index:
>
> postgres=# CREATE TABLE test (id serial, sampledate timestamp);
> CREATE TABLE
> postgres=# INSERT INTO test (sampledate) SELECT '1970-01-01
> 00:00:00'::timestamp + (random()*1350561945 || ' seconds')::interval
> FROM generate_series(1,21000000);
> INSERT 0 21000000
> postgres=# VACUUM;
> VACUUM
> postgres=# EXPLAIN SELECT extract(month FROM sampledate), count(*)
> FROM test GROUP BY extract(month FROM sampledate);
> QUERY PLAN
> ----------------------------------------------------------------------
> HashAggregate (cost=481014.00..481016.50 rows=200 width=8)
> -> Seq Scan on test (cost=0.00..376014.00 rows=21000000 width=8)
> (2 rows)
>
> postgres=# ANALYSE;
> ANALYZE
> postgres=# EXPLAIN SELECT extract(month FROM sampledate), count(*)
> FROM test GROUP BY extract(month FROM sampledate);
> QUERY PLAN
> ----------------------------------------------------------------------------
> GroupAggregate (cost=4078473.42..4498473.90 rows=21000024 width=8)
> -> Sort (cost=4078473.42..4130973.48 rows=21000024 width=8)
> Sort Key: (date_part('month'::text, sampledate))
> -> Seq Scan on test (cost=0.00..376014.30 rows=21000024 width=8)
> (4 rows)
>
> postgres=# CREATE INDEX idx_test_sampledate_month ON test
> (extract(month FROM sampledate));
> CREATE INDEX
> postgres=# EXPLAIN SELECT extract(month FROM sampledate), count(*)
> FROM test GROUP BY extract(month FROM sampledate);
> QUERY PLAN
> ----------------------------------------------------------------------------
> GroupAggregate (cost=4078470.03..4498470.03 rows=21000000 width=8)
> -> Sort (cost=4078470.03..4130970.03 rows=21000000 width=8)
> Sort Key: (date_part('month'::text, sampledate))
> -> Seq Scan on test (cost=0.00..376014.00 rows=21000000 width=8)
> (4 rows)
>
> postgres=# ANALYSE;
> ANALYZE
> postgres=# EXPLAIN SELECT extract(month FROM sampledate), count(*)
> FROM test GROUP BY extract(month FROM sampledate);
> QUERY PLAN
> ----------------------------------------------------------------------
> HashAggregate (cost=481012.85..481013.00 rows=12 width=8)
> -> Seq Scan on test (cost=0.00..376013.17 rows=20999934 width=8)
> (2 rows)
>
>
> The estimate is down to almost a 10th of what it was before. What's going on?
>
> And as a side note, how come it's impossible to get the planner to use
> an index-only scan to satisfy the query (disabling sequential and
> regular index scans)?

I should perhaps mention this is on 9.3devel as of today.

--
Thom