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Fix nasty performance problem in tsquery_rewrite().

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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>
This commit is contained in:
Tom Lane 2016-10-30 17:35:42 -04:00
parent 24ebc444c6
commit 5ec81aceec
1 changed files with 110 additions and 80 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

190
src/backend/utils/adt/tsquery_rewrite.c
View File

@ -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));
bool *matched;
int nmatched;
int i,
j;
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);
/* Assert that the subset rule is OK */
Assert(node->valnode->qoperator.oper == OP_AND ||
node->valnode->qoperator.oper == OP_OR);
for (i = 0; i < ex->nchild; i++)
counters[i] = i;
do
/* 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++;
}
if (QTNEq(tnode, ex))
else if (cmp < 0)
{
int j = 0;
pfree(tnode->valnode);
pfree(tnode->child);
pfree(tnode);
if (subs)
{
tnode = QTNCopy(subs);
tnode->flags = QTN_NOCHANGE | QTN_NEEDFREE;
}
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->nchild = j;
*isfind = true;
/* node->child[i] has no match, ignore it */
i++;
}
else
{
/* ex->child[j] has no match; we can give up immediately */
break;
}
} while (addone(counters, ex->nchild - 1, node->nchild));
if (tnode && (tnode->flags & QTN_NOCHANGE) == 0)
{
pfree(tnode->valnode);
pfree(tnode->child);
pfree(tnode);
}
else
if (nmatched == ex->nchild)
{
/* collapse out the matched children of node */
j = 0;
for (i = 0; i < node->nchild; i++)
{
if (matched[i])
QTNFree(node->child[i]);
else
node->child[j++] = node->child[i];
}
/* and instead insert a copy of subs */
if (subs)
{
subs = QTNCopy(subs);
subs->flags |= QTN_NOCHANGE;
node->child[j++] = subs;
}
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);
pfree(counters);
*isfind = true;
}
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);
}
}