query plan estimate

[pg noob]

Hi all,

I'm looking at this query plan, an excerpt of which is shown here, and I am just wondering how the estimated cost for the Nested Loop is calculated?

->  Nested Loop  (cost=0.00..2888.16 rows=240 width=16) (actual time=0.034..2.180 rows=91 loops=1)
      Output: public.mg.lctime, public.mg.gfid
      ->  Index Scan using _

ba6cf7271af37e26c0e09e3225369f1b on version  (cost=0.00..958.13 rows=240 width=8) (actual time=0.013..0.318 rows=91 loops=1)
            Output: version.id, version.gfid, version.tid, version.seq, version.txsid, version.objectid
      ->  Index Scan using mgfid__uniq on mg  (cost=0.00..8.03 rows=1 width=16) (actual time=0.005..0.008 rows=1 loops=91)
            Output: public.mg.id, public.mg.gfid, public.mg.ftype, public.mg.lctime
            Index Cond: (public.mg.gfid = version.gfid)
            Filter: (public.mg.lctime < 1363076849)

What I expected is that it would be the sum of the output cost of the two index scans?
I have no clue how it came up with 2,888.

Thank you.

[Tom Lane]

pg noob <pgn...@gmail.com> writes:
> I'm looking at this query plan, an excerpt of which is shown here, and I am
> just wondering how the estimated cost for the Nested Loop is calculated?

http://www.postgresql.org/docs/9.2/static/using-explain.html

There's a nestloop example about a third of the way down the page ...

regards, tom lane


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