2012/9/1 Tom Lane <tgl@sss.pgh.pa.us>:
> As implemented in HEAD, LATERAL means to run a nestloop in which the
> lateral-referencing query is run once per row of the referenced table,
> and the resulting rows are joined to just that row of the referenced
> table. So for example:
>
> # select * from (values (2),(4)) v(x), lateral generate_series(1,x);
> x | generate_series
> ---+-----------------
> 2 | 1
> 2 | 2
> 4 | 1
> 4 | 2
> 4 | 3
> 4 | 4
> (6 rows)
>
> It suddenly struck me though that there's another plausible
> interpretation of this syntax: perhaps we should generate all the rows
> of the referencing query as above, and then join them to *all* rows of
> the rest of the query. That is, should the above query generate
>
> x | generate_series
> ---+-----------------
> 2 | 1
> 2 | 1
> 2 | 2
> 2 | 2
> 2 | 3
> 2 | 4
> 4 | 1
> 4 | 1
> 4 | 2
> 4 | 2
> 4 | 3
> 4 | 4
> (12 rows)
>
> This behavior doesn't seem as useful to me --- I think you'd nearly
> always end up adding additional WHERE clauses to get rid of the extra
> rows. However, there should not be any judgment calls involved here;
> this is a spec-defined syntax so surely the SQL standard ought to tell
> us what to do. But I'm darned if I see anything in the standard that
> defines the actual *behavior* of a LATERAL query.
The second variant is really useless - I don't see sense too.
Regards
Pavel
>
> Please point out chapter and verse of what I'm missing. Or, perhaps
> we can hold some committee members' feet to the fire for a ruling?
>
> regards, tom lane
>
>
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