Another regexp performance improvement: skip useless paren-captures

Here's a little finger exercise that improves a case that's bothered me
for awhile.  In a POSIX regexp, parentheses cause capturing by default;
you have to write the very non-obvious "(?:...)" if you don't want the
matching substring to be reported by the regexp engine.  That'd be fine
if capturing were cheap, but with our engine it is not particularly
cheap.  In many situations, the initial DFA check is sufficient to
tell whether there is an overall match, but it does not tell where any
subexpression match boundaries are.  To identify exactly which substring
is deemed to match a parenthesized subexpression, we have to recursively
break down the match, which takes at the very least a few more DFA
invocations; and with an uncooperative regex, it can easily result in
O(N^2) behavior where there was none at the DFA stage.

Therefore, we really ought to expend some effort to not capture
subexpressions if the sub-match data is not actually needed, which in
many invocations we know that it isn't.  Spencer's original code has
a REG_NOSUB option that looks like it ought to be good for this ... but
on closer inspection it's basically useless, because it turns *all*
parens into non-capturing ones.  That breaks back-references, so unless
you know that the regexp contains no back-refs, you can't use it.

The attached proposed patch redefines REG_NOSUB as being a regexp-
compile-time promise that the caller doesn't care about sub-match
locations, but not a promise that no backrefs exist.  (If the
caller passes a match-locations array at execution anyway, it will
just get back -1 values, as if no sub-match had been identified.)
If that flag is passed, we run through the completed sub-regexp
tree and remove the "capture" markers on any subREs that are
not actually referenced by some backref.  This typically causes
some parent subREs to no longer be deemed "messy", so that their
separate child subREs can be thrown away entirely, saving memory
space as well as runtime.

(I'd originally thought that a much more complex patch would be
needed to do this, because I assumed that re-optimizing the subRE
tree would be much more complicated than this.  However, as far
as I can see this is sufficient; this change doesn't expose any
cases where additional tree restructuring would be helpful.)

Testing with Joel's handy little corpus of web regexps, there's a
useful improvement of the speed of ~ operators (a/k/a regexp_like()).
I see the total time to apply regexp_like() to all 4474520 entries
dropping from 10:17 to 5:46.  Interesting statistics include

regexp=# select max(duration),avg(duration) from headresults;
       max       |       avg
-----------------+-----------------
 00:00:00.939389 | 00:00:00.000138
(1 row)

regexp=# select max(duration),avg(duration) from patchresults;
       max       |       avg
-----------------+-----------------
 00:00:00.918549 | 00:00:00.000077
(1 row)

The lower percentiles don't move much, but upper ones do:

regexp=# select percentile_cont(array[0.5,0.75,0.8,0.9]) within group(order by duration) from headresults;
                          percentile_cont
-------------------------------------------------------------------
 {00:00:00.000027,00:00:00.000059,00:00:00.000067,00:00:00.000108}
(1 row)

regexp=# select percentile_cont(array[0.5,0.75,0.8,0.9]) within group(order by duration) from patchresults;
                          percentile_cont
-------------------------------------------------------------------
 {00:00:00.000025,00:00:00.000042,00:00:00.000048,00:00:00.000065}
(1 row)

This isn't terribly surprising, because regexps that were already
really cheap probably have no capturing parens to dispense with.

Of course, there's no benefit with functions that do need sub-match
data, such as regexp_match.  But the added overhead in such cases
should be quite negligible.  The only downside I can see is that
if you use the "same" regexp in both submatches-needed and
non-submatches-needed contexts, you'll end up with two separate
compiled regexp cache entries.  That doesn't seem like a big
problem though.

            regards, tom lane

diff --git a/contrib/pg_trgm/trgm_regexp.c b/contrib/pg_trgm/trgm_regexp.c
index bf1dea6352..64816dd370 100644
--- a/contrib/pg_trgm/trgm_regexp.c
+++ b/contrib/pg_trgm/trgm_regexp.c
@@ -541,9 +541,11 @@ createTrgmNFA(text *text_re, Oid collation,
      * Stage 1: Compile the regexp into a NFA, using the regexp library.
      */
 #ifdef IGNORECASE
-    RE_compile(®ex, text_re, REG_ADVANCED | REG_ICASE, collation);
+    RE_compile(®ex, text_re,
+               REG_ADVANCED | REG_NOSUB | REG_ICASE, collation);
 #else
-    RE_compile(®ex, text_re, REG_ADVANCED, collation);
+    RE_compile(®ex, text_re,
+               REG_ADVANCED | REG_NOSUB, collation);
 #endif

     /*
diff --git a/src/backend/regex/regc_lex.c b/src/backend/regex/regc_lex.c
index 7673dab76f..45727ffa01 100644
--- a/src/backend/regex/regc_lex.c
+++ b/src/backend/regex/regc_lex.c
@@ -528,10 +528,7 @@ next(struct vars *v)
                 }
                 assert(NOTREACHED);
             }
-            if (v->cflags & REG_NOSUB)
-                RETV('(', 0);    /* all parens non-capturing */
-            else
-                RETV('(', 1);
+            RETV('(', 1);
             break;
         case CHR(')'):
             if (LASTTYPE('('))
diff --git a/src/backend/regex/regcomp.c b/src/backend/regex/regcomp.c
index 9f71177d31..38f709cb60 100644
--- a/src/backend/regex/regcomp.c
+++ b/src/backend/regex/regcomp.c
@@ -65,7 +65,7 @@ static struct subre *subre(struct vars *, int, int, struct state *, struct state
 static void freesubre(struct vars *, struct subre *);
 static void freesubreandsiblings(struct vars *, struct subre *);
 static void freesrnode(struct vars *, struct subre *);
-static void optst(struct vars *, struct subre *);
+static void removecaptures(struct vars *, struct subre *);
 static int    numst(struct subre *, int);
 static void markst(struct subre *);
 static void cleanst(struct vars *);
@@ -431,7 +431,8 @@ pg_regcomp(regex_t *re,
         dumpst(v->tree, debug, 1);
     }
 #endif
-    optst(v, v->tree);
+    if (v->cflags & REG_NOSUB)
+        removecaptures(v, v->tree);
     v->ntree = numst(v->tree, 1);
     markst(v->tree);
     cleanst(v);
@@ -1004,14 +1005,16 @@ parseqatom(struct vars *v,
             break;
         case BACKREF:            /* the Feature From The Black Lagoon */
             INSIST(type != LACON, REG_ESUBREG);
-            INSIST(v->nextvalue < v->nsubs, REG_ESUBREG);
-            INSIST(v->subs[v->nextvalue] != NULL, REG_ESUBREG);
+            subno = v->nextvalue;
+            assert(subno > 0);
+            INSIST(subno < v->nsubs, REG_ESUBREG);
+            NOERR();
+            INSIST(v->subs[subno] != NULL, REG_ESUBREG);
             NOERR();
-            assert(v->nextvalue > 0);
             atom = subre(v, 'b', BACKR, lp, rp);
             NOERR();
-            subno = v->nextvalue;
             atom->backno = subno;
+            v->subs[subno]->flags |= BRUSE;
             EMPTYARC(lp, rp);    /* temporarily, so there's something */
             NEXT();
             break;
@@ -2122,19 +2125,54 @@ freesrnode(struct vars *v,        /* might be NULL */
 }

 /*
- * optst - optimize a subRE subtree
+ * removecaptures - remove unnecessary capture subREs
+ *
+ * If the caller said that it doesn't care about subexpression match data,
+ * we may delete the "capture" markers on subREs that are not referenced
+ * by any backrefs, and then simplify anything that's become non-messy.
+ * Call this only if REG_NOSUB flag is set.
  */
 static void
-optst(struct vars *v,
-      struct subre *t)
+removecaptures(struct vars *v,
+               struct subre *t)
 {
+    struct subre *t2;
+
+    assert(t != NULL);
+
+    /*
+     * If this isn't itself a backref target, clear capno and tentatively
+     * clear CAP flag.
+     */
+    if (!(t->flags & BRUSE))
+    {
+        t->capno = 0;
+        t->flags &= ~CAP;
+    }
+
+    /* Now recurse to children */
+    for (t2 = t->child; t2 != NULL; t2 = t2->sibling)
+    {
+        removecaptures(v, t2);
+        /* Propagate child CAP flag back up, if it's still set */
+        if (t2->flags & CAP)
+            t->flags |= CAP;
+    }
+
     /*
-     * DGP (2007-11-13): I assume it was the programmer's intent to eventually
-     * come back and add code to optimize subRE trees, but the routine coded
-     * just spends effort traversing the tree and doing nothing. We can do
-     * nothing with less effort.
+     * If t now contains neither captures nor backrefs, there's no longer any
+     * need to care where its sub-match boundaries are, so we can reduce it to
+     * a simple DFA node.  (Note in particular that MIXED child greediness is
+     * not a hindrance here, so we don't use the MESSY() macro.)
      */
-    return;
+    if ((t->flags & (CAP | BACKR)) == 0)
+    {
+        if (t->child)
+            freesubreandsiblings(v, t->child);
+        t->child = NULL;
+        t->op = '=';
+        t->flags &= ~MIXED;
+    }
 }

 /*
@@ -2482,6 +2520,8 @@ stdump(struct subre *t,
         fprintf(f, " hascapture");
     if (t->flags & BACKR)
         fprintf(f, " hasbackref");
+    if (t->flags & BRUSE)
+        fprintf(f, " isreferenced");
     if (!(t->flags & INUSE))
         fprintf(f, " UNUSED");
     if (t->latype != (char) -1)
diff --git a/src/backend/regex/regexec.c b/src/backend/regex/regexec.c
index 5b9a087820..e75b59e9f0 100644
--- a/src/backend/regex/regexec.c
+++ b/src/backend/regex/regexec.c
@@ -213,12 +213,10 @@ pg_regexec(regex_t *re,
         return REG_NOMATCH;
     backref = (v->g->info & REG_UBACKREF) ? 1 : 0;
     v->eflags = flags;
-    if (v->g->cflags & REG_NOSUB)
-        nmatch = 0;                /* override client */
     v->nmatch = nmatch;
-    if (backref)
+    if (backref && nmatch <= v->g->nsub)
     {
-        /* need work area */
+        /* need larger work area */
         if (v->g->nsub + 1 <= LOCALMAT)
             v->pmatch = mat;
         else
@@ -229,7 +227,13 @@ pg_regexec(regex_t *re,
         v->nmatch = v->g->nsub + 1;
     }
     else
+    {
+        /* we can store results directly in caller's array */
         v->pmatch = pmatch;
+        /* ensure any extra entries in caller's array are filled with -1 */
+        if (nmatch > 0)
+            zapallsubs(pmatch, nmatch);
+    }
     v->details = details;
     v->start = (chr *) string;
     v->search_start = (chr *) string + search_start;
@@ -290,12 +294,20 @@ pg_regexec(regex_t *re,
     else
         st = find(v, &v->g->tree->cnfa, &v->g->cmap);

-    /* copy (portion of) match vector over if necessary */
-    if (st == REG_OKAY && v->pmatch != pmatch && nmatch > 0)
+    /* on success, ensure caller's match vector is filled correctly */
+    if (st == REG_OKAY && nmatch > 0)
     {
-        zapallsubs(pmatch, nmatch);
-        n = (nmatch < v->nmatch) ? nmatch : v->nmatch;
-        memcpy(VS(pmatch), VS(v->pmatch), n * sizeof(regmatch_t));
+        if (v->g->cflags & REG_NOSUB)
+        {
+            /* ensure we don't return partial results */
+            zapallsubs(pmatch, nmatch);
+        }
+        else if (v->pmatch != pmatch)
+        {
+            /* copy portion of match vector over from (larger) work area */
+            assert(nmatch <= v->nmatch);
+            memcpy(VS(pmatch), VS(v->pmatch), nmatch * sizeof(regmatch_t));
+        }
     }

     /* clean up */
diff --git a/src/backend/utils/adt/jsonpath_gram.y b/src/backend/utils/adt/jsonpath_gram.y
index de3d97931e..bd5d4488a0 100644
--- a/src/backend/utils/adt/jsonpath_gram.y
+++ b/src/backend/utils/adt/jsonpath_gram.y
@@ -583,6 +583,14 @@ jspConvertRegexFlags(uint32 xflags)
                      errmsg("XQuery \"x\" flag (expanded regular expressions) is not implemented")));
     }

+    /*
+     * We'll never need sub-match details at execution.  While
+     * RE_compile_and_execute would set this flag anyway, force it on here to
+     * ensure that the regex cache entries created by makeItemLikeRegex are
+     * useful.
+     */
+    cflags |= REG_NOSUB;
+
     return cflags;
 }

diff --git a/src/backend/utils/adt/regexp.c b/src/backend/utils/adt/regexp.c
index 484d4265fd..268cee1cbe 100644
--- a/src/backend/utils/adt/regexp.c
+++ b/src/backend/utils/adt/regexp.c
@@ -347,6 +347,10 @@ RE_compile_and_execute(text *text_re, char *dat, int dat_len,
 {
     regex_t    *re;

+    /* Use REG_NOSUB if caller does not want sub-match details */
+    if (nmatch < 2)
+        cflags |= REG_NOSUB;
+
     /* Compile RE */
     re = RE_compile_and_cache(text_re, cflags, collation);

@@ -1412,6 +1416,7 @@ setup_regexp_matches(text *orig_str, text *pattern, pg_re_flags *re_flags,
     int            orig_len;
     pg_wchar   *wide_str;
     int            wide_len;
+    int            cflags;
     regex_t    *cpattern;
     regmatch_t *pmatch;
     int            pmatch_len;
@@ -1430,7 +1435,10 @@ setup_regexp_matches(text *orig_str, text *pattern, pg_re_flags *re_flags,
     wide_len = pg_mb2wchar_with_len(VARDATA_ANY(orig_str), wide_str, orig_len);

     /* set up the compiled pattern */
-    cpattern = RE_compile_and_cache(pattern, re_flags->cflags, collation);
+    cflags = re_flags->cflags;
+    if (!use_subpatterns)
+        cflags |= REG_NOSUB;
+    cpattern = RE_compile_and_cache(pattern, cflags, collation);

     /* do we want to remember subpatterns? */
     if (use_subpatterns && cpattern->re_nsub > 0)
@@ -1952,7 +1960,7 @@ regexp_fixed_prefix(text *text_re, bool case_insensitive, Oid collation,
     if (case_insensitive)
         cflags |= REG_ICASE;

-    re = RE_compile_and_cache(text_re, cflags, collation);
+    re = RE_compile_and_cache(text_re, cflags | REG_NOSUB, collation);

     /* Examine it to see if there's a fixed prefix */
     re_result = pg_regprefix(re, &str, &slen);
diff --git a/src/include/regex/regex.h b/src/include/regex/regex.h
index 2b48a19fb7..0455ae8069 100644
--- a/src/include/regex/regex.h
+++ b/src/include/regex/regex.h
@@ -106,7 +106,7 @@ typedef struct
 #define REG_QUOTE    000004        /* no special characters, none */
 #define REG_NOSPEC    REG_QUOTE    /* historical synonym */
 #define REG_ICASE    000010        /* ignore case */
-#define REG_NOSUB    000020        /* don't care about subexpressions */
+#define REG_NOSUB    000020        /* caller doesn't need subexpr match data */
 #define REG_EXPANDED    000040    /* expanded format, white space & comments */
 #define REG_NLSTOP    000100        /* \n doesn't match . or [^ ] */
 #define REG_NLANCH    000200        /* ^ matches after \n, $ before */
diff --git a/src/include/regex/regguts.h b/src/include/regex/regguts.h
index 8db631c83b..91a52840c4 100644
--- a/src/include/regex/regguts.h
+++ b/src/include/regex/regguts.h
@@ -477,13 +477,14 @@ struct subre
 #define  MIXED     04                /* mixed preference below */
 #define  CAP     010            /* capturing parens here or below */
 #define  BACKR     020            /* back reference here or below */
+#define  BRUSE     040            /* is referenced by a back reference */
 #define  INUSE     0100            /* in use in final tree */
-#define  NOPROP  03                /* bits which may not propagate up */
+#define  UPPROP  (MIXED|CAP|BACKR)    /* flags which should propagate up */
 #define  LMIX(f) ((f)<<2)        /* LONGER -> MIXED */
 #define  SMIX(f) ((f)<<1)        /* SHORTER -> MIXED */
-#define  UP(f)     (((f)&~NOPROP) | (LMIX(f) & SMIX(f) & MIXED))
+#define  UP(f)     (((f)&UPPROP) | (LMIX(f) & SMIX(f) & MIXED))
 #define  MESSY(f)     ((f)&(MIXED|CAP|BACKR))
-#define  PREF(f) ((f)&NOPROP)
+#define  PREF(f)     ((f)&(LONGER|SHORTER))
 #define  PREF2(f1, f2)     ((PREF(f1) != 0) ? PREF(f1) : PREF(f2))
 #define  COMBINE(f1, f2) (UP((f1)|(f2)) | PREF2(f1, f2))
     char        latype;            /* LATYPE code, if lookaround constraint */
diff --git a/src/test/modules/test_regex/expected/test_regex.out b/src/test/modules/test_regex/expected/test_regex.out
index 01d50ec1e3..dff5a16f57 100644
--- a/src/test/modules/test_regex/expected/test_regex.out
+++ b/src/test/modules/test_regex/expected/test_regex.out
@@ -146,14 +146,21 @@ select * from test_regex('(a)e', 'ae', '-');
  {ae,a}
 (2 rows)

--- expectMatch    4.2  o        (a)e        ae
-select * from test_regex('(a)e', 'ae', 'o');
- test_regex
-------------
- {0}
- {NULL}
+-- expectMatch    4.2  oPR    (.)\1e        aae
+select * from test_regex('(.)\1e', 'aae', 'oPR');
+           test_regex
+--------------------------------
+ {1,REG_UBACKREF,REG_UNONPOSIX}
+ {aae,NULL}
 (2 rows)

+-- expectNomatch    4.2b  oPR    (.)\1e        abe
+select * from test_regex('(.)\1e', 'abe', 'oPR');
+           test_regex
+--------------------------------
+ {1,REG_UBACKREF,REG_UNONPOSIX}
+(1 row)
+
 -- expectMatch    4.3  b        {\(a\)b}    ab    ab    a
 select * from test_regex('\(a\)b', 'ab', 'b');
  test_regex
@@ -2658,6 +2665,14 @@ select * from test_regex('(\w+(?= )).*\1', 'abc abcd abd', 'HLRP');
  {"abc abc",abc}
 (2 rows)

+-- exercise oversize-regmatch_t-array paths in regexec()
+-- (that case is not reachable via test_regex, sadly)
+select substring('fffoooooooooooooooooooooooooooooooo',
'^(.)\1(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)');
+ substring
+-----------
+ f
+(1 row)
+
 -- doing 15 "octal escapes vs back references"
 -- # initial zero is always octal
 -- expectMatch    15.1  MP    "a\\010b"    "a\bb"    "a\bb"
diff --git a/src/test/modules/test_regex/sql/test_regex.sql b/src/test/modules/test_regex/sql/test_regex.sql
index 7f5bc6e418..cfffd13686 100644
--- a/src/test/modules/test_regex/sql/test_regex.sql
+++ b/src/test/modules/test_regex/sql/test_regex.sql
@@ -63,8 +63,10 @@ select * from test_regex('ab', 'ab', 'b');

 -- expectMatch    4.1  -        (a)e        ae    ae    a
 select * from test_regex('(a)e', 'ae', '-');
--- expectMatch    4.2  o        (a)e        ae
-select * from test_regex('(a)e', 'ae', 'o');
+-- expectMatch    4.2  oPR    (.)\1e        aae
+select * from test_regex('(.)\1e', 'aae', 'oPR');
+-- expectNomatch    4.2b  oPR    (.)\1e        abe
+select * from test_regex('(.)\1e', 'abe', 'oPR');
 -- expectMatch    4.3  b        {\(a\)b}    ab    ab    a
 select * from test_regex('\(a\)b', 'ab', 'b');
 -- expectMatch    4.4  -        a((b)c)        abc    abc    bc    b
@@ -775,6 +777,10 @@ select * from test_regex('(^\w+).*\1', 'abc abc abc', 'LRP');
 select * from test_regex('(^\w+\M).*\1', 'abc abcd abd', 'LRP');
 select * from test_regex('(\w+(?= )).*\1', 'abc abcd abd', 'HLRP');

+-- exercise oversize-regmatch_t-array paths in regexec()
+-- (that case is not reachable via test_regex, sadly)
+select substring('fffoooooooooooooooooooooooooooooooo',
'^(.)\1(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)(.)');
+
 -- doing 15 "octal escapes vs back references"

 -- # initial zero is always octal