Re-implement extraction of fixed prefixes from regular expressions.

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.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/include/regex/regex.h b/src/include/regex/regex.h
index cec4b837cd1..616c2c6450d 100644
--- a/src/include/regex/regex.h
+++ b/src/include/regex/regex.h
@@ -156,6 +156,9 @@ typedef struct
 /* two specials for debugging and testing */
 #define REG_ATOI	101			/* convert error-code name to number */
 #define REG_ITOA	102			/* convert error-code number to name */
+/* non-error result codes for pg_regprefix */
+#define REG_PREFIX	(-1)		/* identified a common prefix */
+#define REG_EXACT	(-2)		/* identified an exact match */
 
 
 
@@ -164,6 +167,7 @@ typedef struct
  */
 extern int	pg_regcomp(regex_t *, const pg_wchar *, size_t, int, Oid);
 extern int	pg_regexec(regex_t *, const pg_wchar *, size_t, size_t, rm_detail_t *, size_t, regmatch_t[], int);
+extern int	pg_regprefix(regex_t *, pg_wchar **, size_t *);
 extern void pg_regfree(regex_t *);
 extern size_t pg_regerror(int, const regex_t *, char *, size_t);
 extern void pg_set_regex_collation(Oid collation);