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Comment by tome | Hacker News Reader

Am I right in thinking that this does not increase Postgres's expressive power but allows more concise implementation?

No, you can write queries that are not really possible to express without it. Basically, it allows you to execute a table-valued function for each row in an earlier query.

For example, in SQL Server I find a common use of CROSS APPLY (which appears to be the same thing) is where the "table-valued function" is a SELECT with a WHERE clause referencing the earlier query, an ORDER BY, and a TOP (=LIMIT) 1. (In fact, this is exactly the example given in the article.) It allows you to do things like "for each row in table A, join the last row in table B where NaturalKey(A) = NaturalKey(B) and Value1(A) is greater than or equal to Value2(B)".

Thanks, joining to the last row is an instructive example.

That's not true. Anything you can do with LATERAL you can also do with correlated scalar subqueries in the SELECT list. LATERAL simply makes writing these kinds of queries easier and more intuitive.

The syntax for this is pretty horrible, however. And if you want to return more than one column from the subquery, you would have to duplicate the subquery definition for each column, right? Then you'd have to have faith that the optimizer can work out what you meant and reconstruct just a single subquery.

There's no faith required; the planner is guaranteed not to do that. The "normal" way is to create a composite type containing each of the columns you need, and then "unpack" it to separate columns. Horrible? Yeah, but it's possible.

Is it possible with scalar subqueries to perform anything other than a tree of correlation? With CROSS APPLY one can correlate a DAG of subqueries, e.g. a diamond where B and C depend on A, and D depends on B and C.

What if the limits on the lateral subqueries were 2 instead of 1, and they were doing select * instead on select sum() in the outer query? How would you recreate that with correlated SCALAR subqueries? There's no such thing as non-scalar correlated subqueries is there?

Untested, but this is the general approach:

  SELECT unnest(ar).* FROM
    (SELECT ARRAY(SELECT tbl FROM tbl
                  WHERE .. ORDER BY .. LIMIT 2) AS ar
     FROM .. OFFSET 0) ss;

If you want a specific set of columns instead of *, you'd need to create a custom composite type to create an array of, since it's not possible to "unpack" anonymous records.

Looking at the examples from the official documentation, I agree with your sentiment. Indeed, the conciseness can cause some confusion to people familiar with the existing scoping rules.

IMHO, it's a good thing if LATERAL is only added as some kind of syntactic sugar. I once had to use LATERAL in DB2 as a band-aid solution for its broken scoping rules: https://www.ibm.com/developerworks/mydeveloperworks/blogs/SQ...

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