A lookbehind constraint is like a lookahead constraint in that it consumes
no text; but it checks for existence (or nonexistence) of a match *ending*
at the current point in the string, rather than one *starting* at the
current point. This is a long-requested feature since it exists in many
other regex libraries, but Henry Spencer had never got around to
implementing it in the code we use.
Just making it work is actually pretty trivial; but naive copying of the
logic for lookahead constraints leads to code that often spends O(N^2) time
to scan an N-character string, because we have to run the match engine
from string start to the current probe point each time the constraint is
checked. In typical use-cases a lookbehind constraint will be written at
the start of the regex and hence will need to be checked at every character
--- so O(N^2) work overall. To fix that, I introduced a third copy of the
core DFA matching loop, paralleling the existing longest() and shortest()
loops. This version, matchuntil(), can suspend and resume matching given
a couple of pointers' worth of storage space. So we need only run it
across the string once, stopping at each interesting probe point and then
resuming to advance to the next one.
I also put in an optimization that simplifies one-character lookahead and
lookbehind constraints, such as "(?=x)" or "(?<!\w)", into AHEAD and BEHIND
constraints, which already existed in the engine. This avoids the overhead
of the LACON machinery entirely for these rather common cases.
The net result is that lookbehind constraints run a factor of three or so
slower than Perl's for multi-character constraints, but faster than Perl's
for one-character constraints ... and they work fine for variable-length
constraints, which Perl gives up on entirely. So that's not bad from a
competitive perspective, and there's room for further optimization if
anyone cares. (In reality, raw scan rate across a large input string is
probably not that big a deal for Postgres usage anyway; so I'm happy if
it's linear.)
pg_regprefix was doing nothing with lookahead constraints, which would
be fine if it were the right kind of nothing, but it isn't: we have to
terminate our search for a fixed prefix, not just pretend the LACON arc
isn't there. Otherwise, if the current state has both a LACON outarc and a
single plain-color outarc, we'd falsely conclude that the color represents
an addition to the fixed prefix, and generate an extracted index condition
that restricts the indexscan too much. (See added regression test case.)
Terminating the search is conservative: we could traverse the LACON arc
(thus assuming that the constraint can be satisfied at runtime) and then
examine the outarcs of the linked-to state. But that would be a lot more
work than it seems worth, because writing a LACON followed by a single
plain character is a pretty silly thing to do.
This makes a difference only in rather contrived cases, but it's a bug,
so back-patch to all supported branches.
To generate btree-indexable conditions from regex WHERE conditions (such as
WHERE indexed_col ~ '^foo'), we need to be able to identify any fixed
prefix that a regex might have; that is, find any string that must be a
prefix of all strings satisfying the regex. We used to do that with
entirely ad-hoc code that looked at the source text of the regex. It
didn't know very much about regex syntax, which mostly meant that it would
fail to identify some optimizable cases; but Viktor Rosenfeld reported that
it would produce actively wrong answers for quantified parenthesized
subexpressions, such as '^(foo)?bar'. Rather than trying to extend the
ad-hoc code to cover this, let's get rid of it altogether in favor of
identifying prefixes by examining the compiled form of a regex.
To do this, I've added a new entry point "pg_regprefix" to the regex library;
hopefully it is defined in a sufficiently general fashion that it can remain
in the library when/if that code gets split out as a standalone project.
Since this bug has been there for a very long time, this fix needs to get
back-patched. However it depends on some other recent commits (particularly
the addition of wchar-to-database-encoding conversion), so I'll commit this
separately and then go to work on back-porting the necessary fixes.
