Обсуждение: how does the planer to estimate row when i use order by and group by
hi ,everybody
why does the planer estimate 200 rows when i use order by and group by .
evn:postgresql 8.4 and 9.3
table:
CREATE TABLE a
(
id serial NOT NULL,
name character varying(20),
modifytime timestamp without time zone,
CONSTRAINT a_pk PRIMARY KEY (id)
)
SQL:
explain analyze
select * from
( select id from a order by id ) d
group by id;
Query plan:
"Group (cost=0.15..66.42 rows=200 width=4) (actual time=0.008..0.008 rows=0 loops=1)"
" -> Index Only Scan using a_pk on a (cost=0.15..56.30 rows=810 width=4) (actual time=0.006..0.006 rows=0 loops=1)"
" Heap Fetches: 0"
"Total runtime: 0.046 ms"
Can anybody suggest something or explain this behavior?
On Tue, Aug 12, 2014 at 5:59 AM, 楊新波 <silent0608@gmail.com> wrote: > why does the planer estimate 200 rows when i use order by and group by . > evn:postgresql 8.4 and 9.3 > Can anybody suggest something or explain this behavior? Because the table is empty, analyze doesn't store any stats for the table, so the planner uses some default guesses. This is actually beneficial for cases where you have done some inserts to a new table, and autovacuum hasn't gotten around to analyzing it yet. And it rarely hurts because any query plan will be fast when there's no data. Regards, Marti