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Thread: nested loop joins

nested loop joins

From
Greg Rychlewski
Date:
Hi,

I'm looking to understand how nested loop joins with limits work. Say I have this query

SELECT *
FROM a  INNER JOIN b ON a.id = b.id 
LIMIT 10;

Say this was done as a nested loop join where the rows of "a" are the outer loop. 

In this scenario, would all the rows of "a" be materialized or are they lazily evaluated? 

If they are lazily evaluated is it done using cursors?

Thanks


Re: nested loop joins

From
Tom Lane
Date:
Greg Rychlewski <greg.rychlewski@gmail.com> writes:
> I'm looking to understand how nested loop joins with limits work. Say I
> have this query

> SELECT *
> FROM a  INNER JOIN b ON a.id = b.id
> LIMIT 10;

> Say this was done as a nested loop join where the rows of "a" are the outer
> loop.

> In this scenario, would all the rows of "a" be materialized or are they
> lazily evaluated?

Evaluation of the join will stop as soon as it's produced 10 rows.
Only as much of "a" will be read as necessary to get that.
(Potentially, not all of "b" would be read either, if 10 matches
to the first "a" row exist in "b".)

            regards, tom lane