Thread: Does the SQL standard actually define LATERAL anywhere?
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 | 12 | 24 | 14 | 24 | 34 |
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 | 12 | 12 | 22 | 22 | 32 |
44 | 14 | 14 | 24 | 24 | 34 |
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.
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
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 > > > -- > Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-hackers
On Sat, 2012-09-01 at 00:29 -0400, Tom Lane wrote: > 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. I have it another read and couldn't find anything either. As written, LATERAL is effectively a noise word, AFAICT.
Peter Eisentraut <peter_e@gmx.net> writes:
> On Sat, 2012-09-01 at 00:29 -0400, Tom Lane wrote:
>> 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.
> I have it another read and couldn't find anything either. As written,
> LATERAL is effectively a noise word, AFAICT.
It's not a noise word --- its effects on scope of name visibility are
spelled out clearly enough. What is not clear is what the query
semantics are supposed to be in the presence of references that would
be disallowed in the traditional understanding of a FROM clause.
regards, tom lane