Re: Another regexp performance improvement: skip useless paren-captures

Mark Dilger <mark(dot)dilger(at)enterprisedb(dot)com> writes:
> +select regexp_split_to_array('', '(?:((?:q+))){0}(\1){0,0}?*[^]');
> +server closed the connection unexpectedly

Here's a quick draft patch for this. Basically it moves the
responsibility for clearing v->subs[] pointers into the freesubre()
recursion, so that it will happen for contained capturing parens
not only the top level.

There is a potentially interesting definitional question:
what exactly ought this regexp do?

((.)){0}\2

Because the capturing paren sets are zero-quantified, they will
never be matched to any characters, so the backref can never
have any defined referent. I suspect that study of the POSIX
spec would lead to the conclusion that this is a legal regexp
but it will never match anything. Implementing that would be
tedious though, and what's more it seems very unlikely that
the user wanted any such behavior. So I think throwing an
error is an appropriate response. The existing code will
throw such an error for

((.)){0}\1

so I guess Spencer did think about this to some extent -- he
just forgot about the possibility of nested parens.

This patch should work OK in HEAD and v14, but it will need
a bit of fooling-about for older branches I think, given that
they fill v->subs[] a little differently.

regards, tom lane