From 92ee2528d8bf46739a460e3395057416c53e10d1 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Fri, 22 Aug 2003 20:26:43 +0000
Subject: [PATCH] Tweak processing of multiple-index-scan plans to reduce
overhead when handling many-way scans: instead of re-evaluating all prior
indexscan quals to see if a tuple has been fetched more than once, use a hash
table indexed by tuple CTID. But fall back to the old way if the hash table
grows to exceed SortMem.
---
src/backend/executor/nodeIndexscan.c | 178 ++++++++++++++++++++++-----
src/include/nodes/execnodes.h | 6 +-
2 files changed, 153 insertions(+), 31 deletions(-)
diff --git a/src/backend/executor/nodeIndexscan.c b/src/backend/executor/nodeIndexscan.c
index bfbcaf208de..f93802269da 100644
--- a/src/backend/executor/nodeIndexscan.c
+++ b/src/backend/executor/nodeIndexscan.c
@@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
- * $Header: /cvsroot/pgsql/src/backend/executor/nodeIndexscan.c,v 1.82 2003/08/04 02:39:59 momjian Exp $
+ * $Header: /cvsroot/pgsql/src/backend/executor/nodeIndexscan.c,v 1.83 2003/08/22 20:26:43 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -28,19 +28,51 @@
#include "access/heapam.h"
#include "executor/execdebug.h"
#include "executor/nodeIndexscan.h"
+#include "miscadmin.h"
#include "nodes/nodeFuncs.h"
#include "optimizer/clauses.h"
#include "parser/parsetree.h"
-/* ----------------
- * Misc stuff to move to executor.h soon -cim 6/5/90
- * ----------------
- */
+
#define NO_OP 0
#define LEFT_OP 1
#define RIGHT_OP 2
+/*
+ * In a multiple-index plan, we must take care to return any given tuple
+ * only once, even if it matches conditions of several index scans. Our
+ * preferred way to do this is to record already-returned tuples in a hash
+ * table (using the TID as unique identifier). However, in a very large
+ * scan this could conceivably run out of memory. We limit the hash table
+ * to no more than SortMem KB; if it grows past that, we fall back to the
+ * pre-7.4 technique: evaluate the prior-scan index quals again for each
+ * tuple (which is space-efficient, but slow).
+ *
+ * When scanning backwards, we use scannum to determine when to emit the
+ * tuple --- we have to re-emit a tuple in the same scan as it was first
+ * encountered.
+ *
+ * Note: this code would break if the planner were ever to create a multiple
+ * index plan with overall backwards direction, because the hashtable code
+ * will emit a tuple the first time it is encountered (which would be the
+ * highest scan in which it matches the index), but the evaluate-the-quals
+ * code will emit a tuple in the lowest-numbered scan in which it's valid.
+ * This could be fixed at need by making the evaluate-the-quals case more
+ * complex. Currently the planner will never create such a plan (since it
+ * considers multi-index plans unordered anyway), so there's no need for
+ * more complexity.
+ */
+typedef struct
+{
+ /* tid is the hash key and so must be first! */
+ ItemPointerData tid; /* TID of a tuple we've returned */
+ int scannum; /* number of scan we returned it in */
+} DupHashTabEntry;
+
+
static TupleTableSlot *IndexNext(IndexScanState *node);
+static void create_duphash(IndexScanState *node);
+
/* ----------------------------------------------------------------
* IndexNext
@@ -163,7 +195,7 @@ IndexNext(IndexScanState *node)
while ((tuple = index_getnext(scandesc, direction)) != NULL)
{
/*
- * store the scanned tuple in the scan tuple slot of the scan
+ * Store the scanned tuple in the scan tuple slot of the scan
* state. Note: we pass 'false' because tuples returned by
* amgetnext are pointers onto disk pages and must not be
* pfree()'d.
@@ -174,36 +206,80 @@ IndexNext(IndexScanState *node)
false); /* don't pfree */
/*
- * We must check to see if the current tuple was already
- * matched by an earlier index, so we don't double-report it.
- * We do this by passing the tuple through ExecQual and
- * checking for failure with all previous qualifications.
+ * If it's a multiple-index scan, make sure not to double-report
+ * a tuple matched by more than one index. (See notes above.)
*/
- if (node->iss_IndexPtr > 0)
+ if (numIndices > 1)
{
- bool prev_matches = false;
- int prev_index;
- List *qual;
-
- econtext->ecxt_scantuple = slot;
- ResetExprContext(econtext);
- qual = node->indxqualorig;
- for (prev_index = 0;
- prev_index < node->iss_IndexPtr;
- prev_index++)
+ /* First try the hash table */
+ if (node->iss_DupHash)
{
- if (ExecQual((List *) lfirst(qual), econtext, false))
+ DupHashTabEntry *entry;
+ bool found;
+
+ entry = (DupHashTabEntry *)
+ hash_search(node->iss_DupHash,
+ &tuple->t_data->t_ctid,
+ HASH_ENTER,
+ &found);
+ if (entry == NULL ||
+ node->iss_DupHash->hctl->nentries > node->iss_MaxHash)
+ {
+ /* out of memory (either hard or soft limit) */
+ /* release hash table and fall thru to old code */
+ hash_destroy(node->iss_DupHash);
+ node->iss_DupHash = NULL;
+ }
+ else if (found)
{
- prev_matches = true;
- break;
+ /* pre-existing entry */
+
+ /*
+ * It's duplicate if first emitted in a different
+ * scan. If same scan, we must be backing up, so
+ * okay to emit again.
+ */
+ if (entry->scannum != node->iss_IndexPtr)
+ {
+ /* Dup, so drop it and loop back for another */
+ ExecClearTuple(slot);
+ continue;
+ }
+ }
+ else
+ {
+ /* new entry, finish filling it in */
+ entry->scannum = node->iss_IndexPtr;
}
- qual = lnext(qual);
}
- if (prev_matches)
+ /* If hash table has overflowed, do it the hard way */
+ if (node->iss_DupHash == NULL &&
+ node->iss_IndexPtr > 0)
{
- /* Duplicate, so drop it and loop back for another */
- ExecClearTuple(slot);
- continue;
+ bool prev_matches = false;
+ int prev_index;
+ List *qual;
+
+ econtext->ecxt_scantuple = slot;
+ ResetExprContext(econtext);
+ qual = node->indxqualorig;
+ for (prev_index = 0;
+ prev_index < node->iss_IndexPtr;
+ prev_index++)
+ {
+ if (ExecQual((List *) lfirst(qual), econtext, false))
+ {
+ prev_matches = true;
+ break;
+ }
+ qual = lnext(qual);
+ }
+ if (prev_matches)
+ {
+ /* Dup, so drop it and loop back for another */
+ ExecClearTuple(slot);
+ continue;
+ }
}
}
@@ -383,6 +459,14 @@ ExecIndexReScan(IndexScanState *node, ExprContext *exprCtxt)
return;
}
+ /* reset hash table */
+ if (numIndices > 1)
+ {
+ if (node->iss_DupHash)
+ hash_destroy(node->iss_DupHash);
+ create_duphash(node);
+ }
+
/* reset index scans */
if (ScanDirectionIsBackward(((IndexScan *) node->ss.ps.plan)->indxorderdir))
node->iss_IndexPtr = numIndices;
@@ -432,6 +516,10 @@ ExecEndIndexScan(IndexScanState *node)
ExecClearTuple(node->ss.ps.ps_ResultTupleSlot);
ExecClearTuple(node->ss.ss_ScanTupleSlot);
+ /* drop hash table */
+ if (node->iss_DupHash)
+ hash_destroy(node->iss_DupHash);
+
/*
* close the index relations
*/
@@ -507,7 +595,7 @@ ExecIndexRestrPos(IndexScanState *node)
/* ----------------------------------------------------------------
* ExecInitIndexScan
- *
+ *
* Initializes the index scan's state information, creates
* scan keys, and opens the base and index relations.
*
@@ -919,12 +1007,42 @@ ExecInitIndexScan(IndexScan *node, EState *estate)
ExecAssignResultTypeFromTL(&indexstate->ss.ps);
ExecAssignScanProjectionInfo(&indexstate->ss);
+ /*
+ * Initialize hash table if needed.
+ */
+ if (numIndices > 1)
+ create_duphash(indexstate);
+ else
+ indexstate->iss_DupHash = NULL;
+
/*
* all done.
*/
return indexstate;
}
+static void
+create_duphash(IndexScanState *node)
+{
+ HASHCTL hash_ctl;
+
+ MemSet(&hash_ctl, 0, sizeof(hash_ctl));
+ hash_ctl.keysize = SizeOfIptrData;
+ hash_ctl.entrysize = sizeof(DupHashTabEntry);
+ hash_ctl.hash = tag_hash;
+ hash_ctl.hcxt = CurrentMemoryContext;
+ node->iss_DupHash = hash_create("DupHashTable",
+ (long) ceil(node->ss.ps.plan->plan_rows),
+ &hash_ctl,
+ HASH_ELEM | HASH_FUNCTION | HASH_CONTEXT);
+ if (node->iss_DupHash == NULL)
+ ereport(ERROR,
+ (errcode(ERRCODE_OUT_OF_MEMORY),
+ errmsg("out of memory")));
+ node->iss_MaxHash = (SortMem * 1024L) /
+ (MAXALIGN(sizeof(HASHELEMENT)) + MAXALIGN(sizeof(DupHashTabEntry)));
+}
+
int
ExecCountSlotsIndexScan(IndexScan *node)
{
diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index 8d180009bfd..97609f58385 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -7,7 +7,7 @@
* Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
- * $Id: execnodes.h,v 1.104 2003/08/19 01:13:41 tgl Exp $
+ * $Id: execnodes.h,v 1.105 2003/08/22 20:26:43 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -768,6 +768,8 @@ typedef ScanState SeqScanState;
* RuntimeKeysReady true if runtime Skeys have been computed
* RelationDescs ptr to array of relation descriptors
* ScanDescs ptr to array of scan descriptors
+ * DupHash hashtable for recognizing dups in multiple scan
+ * MaxHash max # entries we will allow in hashtable
* ----------------
*/
typedef struct IndexScanState
@@ -785,6 +787,8 @@ typedef struct IndexScanState
bool iss_RuntimeKeysReady;
RelationPtr iss_RelationDescs;
IndexScanDescPtr iss_ScanDescs;
+ HTAB *iss_DupHash;
+ long iss_MaxHash;
} IndexScanState;
/* ----------------
--
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