From: Andrew Gierth <andrew@tao11.riddles.org.uk>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@lists.postgresql.org
Subject: Re: SQL-spec incompatibilities in similar_escape() and related stuff
In-Reply-To: <4486.1557769997@sss.pgh.pa.us> (Tom Lane's message of "Mon, 13
	May 2019 13:53:17 -0400")
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Date: Mon, 13 May 2019 19:39:14 +0100
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>>>>> "Tom" == Tom Lane <tgl@sss.pgh.pa.us> writes:

 Tom> Hmm.  Oddly, you can't fix it by adding parens:

 Tom> regression=# select 'foo' similar to ('f' || escape) escape escape from (values ('oo')) v(escape);
 Tom> psql: ERROR:  syntax error at or near "escape"
 Tom> LINE 1: select 'foo' similar to ('f' || escape) escape escape from (...
 Tom>                                         ^

 Tom> Since "escape" is an unreserved word, I'd have expected that to
 Tom> work. Odd.

Simpler cases fail too:

select 'f' || escape from (values ('o')) v(escape);
psql: ERROR:  syntax error at or near "escape"

select 1 + escape from (values (1)) v(escape);  -- works
select 1 & escape from (values (1)) v(escape);  -- fails

in short ESCAPE can't follow any generic operator, because its lower
precedence forces the operator to be reduced as a postfix op instead.

 Tom> The big picture here is that fixing grammar ambiguities by adding
 Tom> precedence is a dangerous business :-(

Yeah. But the alternative is usually reserving words more strictly,
which has its own issues :-(

Or we could kill off postfix operators...

-- 
Andrew (irc:RhodiumToad)