Re: Doubts on startup costs - Mailing list pgsql-general

Start-up cost is the cost required to fetch the first tuple. So yes it is possible for the startup cost of innermost node/leaf node to be zero as show in below example:.

postgres=# explain select * from tbl,tbl2  where tbl2.id=tbl.id order by tbl.id;

                                    QUERY PLAN

----------------------------------------------------------------------------------

Merge Join  (cost=809.81..1636.50 rows=11571 width=8)

   Merge Cond: (tbl.id = tbl2.id)

   ->  Index Only Scan using idx on tbl  (cost=0.42..3369.01 rows=110000 width=4)

   ->  Sort  (cost=809.39..834.39 rows=10000 width=4)

         Sort Key: tbl2.id

         ->  Seq Scan on tbl2  (cost=0.00..145.00 rows=10000 width=4)

Planning time: 0.672 ms

postgres=# explain select * from tbl order by id;

                                 QUERY PLAN

----------------------------------------------------------------------------

Index Only Scan using idx on tbl  (cost=0.42..3369.01 rows=110000 width=4)

Planning time: 0.305 ms

Also start-up cost of outer node need not be more than total cost of inner nodes. If it is possible for outer nodes to

emit one tuple without waiting for complete operation to happen by inner nodes, then outer node start-up cost will be

much lesser than total cost by inner nodes. But start-up cost of outer node cannot be less the start-up cost of inner nodes.

For example in above example plan, a merge join can emit one tuple as soon as it finds one matching row.

So start-up cost is lesser.

Thanks and Regards,

Kumar Rajeev Rastogi

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