Fix nasty performance problem in tsquery_rewrite().

tsquery_rewrite() tries to find matches to subsets of AND/OR conditions;
for example, in the query 'a | b | c' the substitution subquery 'a | c'
should match and lead to replacement of the first and third items.
That's fine, but the matching algorithm apparently takes about O(2^N)
for an N-clause query (I say "apparently" because the code is also both
unintelligible and uncommented).  We could probably do better than that
even without any extra assumptions --- but actually, we know that the
subclauses are sorted, indeed are depending on that elsewhere in this very
same function.  So we can just scan the two lists a single time to detect
matches, as though we were doing a merge join.

Also do a re-flattening call (QTNTernary()) in tsquery_rewrite_query, just
to make sure that the tree fits the expectations of the next search cycle.
I didn't try to devise a test case for this, but I'm pretty sure that the
oversight could have led to failure to match in some cases where a match
would be expected.

Improve comments, and also stick a CHECK_FOR_INTERRUPTS into
dofindsubquery, just in case it's still too slow for somebody.

Per report from Andreas Seltenreich.  Back-patch to all supported branches.

Discussion: <8760oasf2y.fsf@credativ.de>

src/backend/utils/adt/tsquery_rewrite.c

@@ -21,47 +21,43 @@
 #include "utils/builtins.h"
 
 
-static int
-addone(int *counters, int last, int total)
-{
-	/* since this function recurses, it could be driven to stack overflow. */
-	check_stack_depth();
-
-	counters[last]++;
-	if (counters[last] >= total)
-	{
-		if (last == 0)
-			return 0;
-		if (addone(counters, last - 1, total - 1) == 0)
-			return 0;
-		counters[last] = counters[last - 1] + 1;
-	}
-	return 1;
-}
-
 /*
- * If node is equal to ex, replace it with subs. Replacement is actually done
- * by returning either node or a copy of subs.
+ * If "node" is equal to "ex", return a copy of "subs" instead.
+ * If "ex" matches a subset of node's children, return a modified version
+ * of "node" in which those children are replaced with a copy of "subs".
+ * Otherwise return "node" unmodified.
+ *
+ * The QTN_NOCHANGE bit is set in successfully modified nodes, so that
+ * we won't uselessly recurse into them.
+ * Also, set *isfind true if we make a replacement.
  */
 static QTNode *
 findeq(QTNode *node, QTNode *ex, QTNode *subs, bool *isfind)
 {
+	/* Can't match unless signature matches and node type matches. */
 	if ((node->sign & ex->sign) != ex->sign ||
 		node->valnode->type != ex->valnode->type)
 		return node;
 
+	/* Ignore nodes marked NOCHANGE, too. */
 	if (node->flags & QTN_NOCHANGE)
 		return node;
 
 	if (node->valnode->type == QI_OPR)
 	{
+		/* Must be same operator. */
 		if (node->valnode->qoperator.oper != ex->valnode->qoperator.oper)
 			return node;
 
 		if (node->nchild == ex->nchild)
 		{
+			/*
+			 * Simple case: when same number of children, match if equal.
+			 * (This is reliable when the children were sorted earlier.)
+			 */
 			if (QTNEq(node, ex))
 			{
+				/* Match; delete node and return a copy of subs instead. */
 				QTNFree(node);
 				if (subs)
 				{
@@ -73,79 +69,92 @@ findeq(QTNode *node, QTNode *ex, QTNode *subs, bool *isfind)
 				*isfind = true;
 			}
 		}
-		else if (node->nchild > ex->nchild)
+		else if (node->nchild > ex->nchild && ex->nchild > 0)
 		{
 			/*
-			 * AND and NOT are commutative, so we check if a subset of the
-			 * children match. For example, if tnode is A | B | C, and ex is B
-			 * | C, we have a match after we convert tnode to A | (B | C).
+			 * AND and OR are commutative/associative, so we should check if a
+			 * subset of the children match.  For example, if node is A|B|C,
+			 * and ex is B|C, we have a match after we notionally convert node
+			 * to A|(B|C).  This does not work for NOT or PHRASE nodes, but we
+			 * can't get here for those node types because they have a fixed
+			 * number of children.
+			 *
+			 * Because we expect that the children are sorted, it suffices to
+			 * make one pass through the two lists to find the matches.
 			 */
-			int		   *counters = (int *) palloc(sizeof(int) * node->nchild);
-			int			i;
-			QTNode	   *tnode = (QTNode *) palloc(sizeof(QTNode));
-
-			memset(tnode, 0, sizeof(QTNode));
-			tnode->child = (QTNode **) palloc(sizeof(QTNode *) * ex->nchild);
-			tnode->nchild = ex->nchild;
-			tnode->valnode = (QueryItem *) palloc(sizeof(QueryItem));
-			*(tnode->valnode) = *(ex->valnode);
-
-			for (i = 0; i < ex->nchild; i++)
-				counters[i] = i;
-
-			do
+			bool	   *matched;
+			int			nmatched;
+			int			i,
+						j;
+
+			/* Assert that the subset rule is OK */
+			Assert(node->valnode->qoperator.oper == OP_AND ||
+				   node->valnode->qoperator.oper == OP_OR);
+
+			/* matched[] will record which children of node matched */
+			matched = (bool *) palloc0(node->nchild * sizeof(bool));
+			nmatched = 0;
+			i = j = 0;
+			while (i < node->nchild && j < ex->nchild)
 			{
-				tnode->sign = 0;
-				for (i = 0; i < ex->nchild; i++)
+				int			cmp = QTNodeCompare(node->child[i], ex->child[j]);
+
+				if (cmp == 0)
 				{
-					tnode->child[i] = node->child[counters[i]];
-					tnode->sign |= tnode->child[i]->sign;
+					/* match! */
+					matched[i] = true;
+					nmatched++;
+					i++, j++;
 				}
+				else if (cmp < 0)
+				{
+					/* node->child[i] has no match, ignore it */
+					i++;
+				}
+				else
+				{
+					/* ex->child[j] has no match; we can give up immediately */
+					break;
+				}
+			}
 
-				if (QTNEq(tnode, ex))
+			if (nmatched == ex->nchild)
+			{
+				/* collapse out the matched children of node */
+				j = 0;
+				for (i = 0; i < node->nchild; i++)
 				{
-					int			j = 0;
-
-					pfree(tnode->valnode);
-					pfree(tnode->child);
-					pfree(tnode);
-					if (subs)
-					{
-						tnode = QTNCopy(subs);
-						tnode->flags = QTN_NOCHANGE | QTN_NEEDFREE;
-					}
+					if (matched[i])
+						QTNFree(node->child[i]);
 					else
-						tnode = NULL;
-
-					node->child[counters[0]] = tnode;
-
-					for (i = 1; i < ex->nchild; i++)
-						node->child[counters[i]] = NULL;
-					for (i = 0; i < node->nchild; i++)
-					{
-						if (node->child[i])
-						{
-							node->child[j] = node->child[i];
-							j++;
-						}
-					}
+						node->child[j++] = node->child[i];
+				}
 
-					node->nchild = j;
+				/* and instead insert a copy of subs */
+				if (subs)
+				{
+					subs = QTNCopy(subs);
+					subs->flags |= QTN_NOCHANGE;
+					node->child[j++] = subs;
+				}
 
-					*isfind = true;
+				node->nchild = j;
+
+				/*
+				 * Re-sort the node to put new child in the right place.  This
+				 * is a bit bogus, because it won't matter for findsubquery's
+				 * remaining processing, and it's insufficient to prepare the
+				 * tree for another search (we would need to re-flatten as
+				 * well, and we don't want to do that because we'd lose the
+				 * QTN_NOCHANGE marking on the new child).  But it's needed to
+				 * keep the results the same as the regression tests expect.
+				 */
+				QTNSort(node);
 
-					break;
-				}
-			} while (addone(counters, ex->nchild - 1, node->nchild));
-			if (tnode && (tnode->flags & QTN_NOCHANGE) == 0)
-			{
-				pfree(tnode->valnode);
-				pfree(tnode->child);
-				pfree(tnode);
+				*isfind = true;
 			}
-			else
-				QTNSort(node);
-			pfree(counters);
+
+			pfree(matched);
 		}
 	}
 	else
@@ -173,12 +182,20 @@ findeq(QTNode *node, QTNode *ex, QTNode *subs, bool *isfind)
 	return node;
 }
 
+/*
+ * Recursive guts of findsubquery(): attempt to replace "ex" with "subs"
+ * at the root node, and if we failed to do so, recursively match against
+ * child nodes.
+ */
 static QTNode *
 dofindsubquery(QTNode *root, QTNode *ex, QTNode *subs, bool *isfind)
 {
 	/* since this function recurses, it could be driven to stack overflow. */
 	check_stack_depth();
 
+	/* also, since it's a bit expensive, let's check for query cancel. */
+	CHECK_FOR_INTERRUPTS();
+
 	root = findeq(root, ex, subs, isfind);
 
 	if (root && (root->flags & QTN_NOCHANGE) == 0 && root->valnode->type == QI_OPR)
@@ -192,6 +209,10 @@ dofindsubquery(QTNode *root, QTNode *ex, QTNode *subs, bool *isfind)
 	return root;
 }
 
+/*
+ * Delete any void subtrees that may have been inserted when the replacement
+ * subtree is void.
+ */
 static QTNode *
 dropvoidsubtree(QTNode *root)
 {
@@ -231,6 +252,14 @@ dropvoidsubtree(QTNode *root)
 	return root;
 }
 
+/*
+ * Substitute "subs" for "ex" throughout the QTNode tree at root.
+ *
+ * If isfind isn't NULL, set *isfind to show whether we made any substitution.
+ *
+ * Both "root" and "ex" must have been through QTNTernary and QTNSort
+ * to ensure reliable matching.
+ */
 QTNode *
 findsubquery(QTNode *root, QTNode *ex, QTNode *subs, bool *isfind)
 {
@@ -344,6 +373,7 @@ tsquery_rewrite_query(PG_FUNCTION_ARGS)
 				{
 					/* ready the tree for another pass */
 					QTNClearFlags(tree, QTN_NOCHANGE);
+					QTNTernary(tree);
 					QTNSort(tree);
 				}
 			}