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Commit57a2d4a

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Fix performance bug in regexp's citerdissect/creviterdissect.
After detecting a sub-match "dissect" failure (i.e., a backref matchfailure) in the i'th sub-match of an iteration node, we should proceedby adjusting the attempted length of the i'th submatch. As coded,though, these functions changed the attempted length of the *last*sub-match, and only after exhausting all possibilities for that wouldthey back up to adjust the next-to-last sub-match, and then thesecond-from-last, etc; all of which is wasted effort, since onlychanging the start or length of the i'th sub-match can possibly makeit succeed. This oversight creates the possibility for exponentiallybad performance. Fortunately the problem is masked in most cases byoptimizations or constraints applied elsewhere; which explains whywe'd not noticed it before. But it is possible to reach the problemwith fairly simple, if contrived, regexps.Oversight in my commit173e29a. That's pretty ancient now,so back-patch to all supported branches.Discussion:https://postgr.es/m/1808998.1629412269@sss.pgh.pa.us
1 parent92ce7f5 commit57a2d4a

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‎src/backend/regex/regexec.c‎

Lines changed: 10 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -1133,8 +1133,8 @@ citerdissect(struct vars *v,
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* Our strategy is to first find a set of sub-match endpoints that are
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* valid according to the child node's DFA, and then recursively dissect
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* each sub-match to confirm validity. If any validity check fails,
1136-
* backtrackthe lastsub-match and try again. And, when we next try for
1137-
*avalidity check, we need not recheck any successfully verified
1136+
* backtrackthatsub-match and try again. And, when we next try for a
1137+
* validity check, we need not recheck any successfully verified
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* sub-matches that we didn't move the endpoints of. nverified remembers
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* how many sub-matches are currently known okay.
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*/
@@ -1222,12 +1222,13 @@ citerdissect(struct vars *v,
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returnREG_OKAY;
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}
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1225-
/* match failed to verify, so backtrack */
1225+
/* i'th match failed to verify, so backtrack it */
1226+
k=i;
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backtrack:
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/*
1230-
* Must consider shorter versions of thecurrent sub-match. However,
1231+
* Must consider shorter versions of thek'th sub-match. However,
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* we'll only ask for a zero-length match if necessary.
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*/
12331234
while (k>0)
@@ -1338,8 +1339,8 @@ creviterdissect(struct vars *v,
13381339
* Our strategy is to first find a set of sub-match endpoints that are
13391340
* valid according to the child node's DFA, and then recursively dissect
13401341
* each sub-match to confirm validity. If any validity check fails,
1341-
* backtrackthe lastsub-match and try again. And, when we next try for
1342-
*avalidity check, we need not recheck any successfully verified
1342+
* backtrackthatsub-match and try again. And, when we next try for a
1343+
* validity check, we need not recheck any successfully verified
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* sub-matches that we didn't move the endpoints of. nverified remembers
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* how many sub-matches are currently known okay.
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*/
@@ -1433,12 +1434,13 @@ creviterdissect(struct vars *v,
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returnREG_OKAY;
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}
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1436-
/* match failed to verify, so backtrack */
1437+
/* i'th match failed to verify, so backtrack it */
1438+
k=i;
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backtrack:
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/*
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* Must consider longer versions of thecurrent sub-match.
1443+
* Must consider longer versions of thek'th sub-match.
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*/
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while (k>0)
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{

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