Re: WIP patch for LATERAL subqueries

Andrew Dunstan <andrew@dunslane.net> writes:
> On 08/05/2012 05:58 PM, Tom Lane wrote:
>> Currently the patch only implements the syntax called out in the standard,
>> namely that you can put LATERAL in front of a <derived table>, which is
>> to say a parenthesized sub-SELECT in FROM.  It strikes me that it might be
>> worth allowing LATERAL with a function-in-FROM as well.

> Pro. As you say this is the main use case, and the longer syntax just 
> seems unnecessary fluff.

After some experimentation it seems that this only works if we promote
LATERAL to a fully reserved keyword.  Apparently the reason is that
given non-reserved LATERAL followed by an identifier, it's not clear
without additional lookahead whether we have "LATERAL func_name ..."
or the LATERAL is a table name and the identifier is an alias.  And the
parser has to make a shift/reduce decision before it can look beyond the
identifier.  (Without the LATERAL func_name syntax, there's no ambiguity
because LATERAL in its keyword meaning must be immediately followed by a
left paren.)

Since LATERAL has been a reserved word in every SQL spec since SQL:99,
I don't feel too bad about making it fully reserved for us too, but
nonetheless this is a cost of adding this syntax.
        regards, tom lane