Re: [HACKERS] What does explain show ?

> Table a has 15905 rows and table b has 25905 rows.
>
>
> For the following query
>
> select a.pkey, b.key2 from a, b
> where b.key1 = 1369
> and a.pkey = b.key1;
>
> "explain" shows
>
> NOTICE: QUERY PLAN:
>
> Nested Loop (cost=6.19 rows=3 width=10)
> -> Index Scan using b_pkey on b on b (cost=2.09 rows=2 width=6)
> -> Index Scan using a_pkey on a on a (cost=2.05 rows=15905 width=4)
>
> What does "rows=15905" of InnerPlan mean ?
> Is "rows=3" of Nested Loop irrelevant to "rows=15905" ?

It means it thinks it is going to access X rows in that pass, but end
up with 3 joined rows as a result of the nested loop. It is only an
estimate, based on table size and column uniqueness from vacuum analyze.

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