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Optimize queries of the form "SELECT count(*) FROM <tbl>" by adding a sqlite3BtreeCount() interface to the btree layer. (CVS 6316)

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FossilOrigin-Name: d4aa6593183224b6868a322511511c0bbf63b598
This commit is contained in:
danielk1977
2009-02-24 10:01:51 +00:00
parent e2d7b24d08
commit a55331620e
9 changed files with 259 additions and 104 deletions
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206
src/select.c
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@@ -12,7 +12,7 @@
** This file contains C code routines that are called by the parser
** to handle SELECT statements in SQLite.
**
** $Id: select.c,v 1.501 2009/02/20 10:58:42 danielk1977 Exp $
** $Id: select.c,v 1.502 2009/02/24 10:01:52 danielk1977 Exp $
*/
#include "sqliteInt.h"
@@ -2953,6 +2953,39 @@ static u8 minMaxQuery(Select *p){
return WHERE_ORDERBY_NORMAL;
}
/*
** The select statement passed as the first argument is an aggregate query.
** The second argment is the associated aggregate-info object. This
** function tests if the SELECT is of the form:
**
** SELECT count(*) FROM <tbl>
**
** where table is a database table, not a sub-select or view. If the query
** does match this pattern, then a pointer to the Table object representing
** <tbl> is returned. Otherwise, 0 is returned.
*/
static Table *isSimpleCount(Select *p, AggInfo *pAggInfo){
Table *pTab;
Expr *pExpr;
assert( !p->pGroupBy );
if( p->pWhere || p->pHaving || p->pEList->nExpr!=1
|| p->pSrc->nSrc!=1 || p->pSrc->a[0].pSelect
){
return 0;
}
pTab = p->pSrc->a[0].pTab;
pExpr = p->pEList->a[0].pExpr;
if( !pTab || pTab->pSelect || IsVirtual(pTab) ) return 0;
if( pExpr->op!=TK_AGG_FUNCTION ) return 0;
if( (pAggInfo->aFunc[0].pFunc->flags&SQLITE_FUNC_COUNT)==0 ) return 0;
if( pExpr->flags&EP_Distinct ) return 0;
return pTab;
}
/*
** If the source-list item passed as an argument was augmented with an
** INDEXED BY clause, then try to locate the specified index. If there
@@ -3998,70 +4031,127 @@ int sqlite3Select(
} /* endif pGroupBy */
else {
ExprList *pMinMax = 0;
ExprList *pDel = 0;
u8 flag;
#ifndef SQLITE_OMIT_BTREECOUNT
Table *pTab;
if( (pTab = isSimpleCount(p, &sAggInfo))!=0 ){
/* If isSimpleCount() returns a pointer to a Table structure, then
** the SQL statement is of the form:
**
** SELECT count(*) FROM <tbl>
**
** where the Table structure returned represents table <tbl>.
**
** This statement is so common that it is optimized specially. The
** OP_Count instruction is executed either on the intkey table that
** contains the data for table <tbl> or on one of its indexes. It
** is better to execute the op on an index, as indexes are almost
** always spread across less pages than their corresponding tables.
*/
const int iDb = sqlite3SchemaToIndex(pParse->db, pTab->pSchema);
const int iCsr = pParse->nTab++; /* Cursor to scan b-tree */
Index *pIdx; /* Iterator variable */
KeyInfo *pKeyInfo = 0; /* Keyinfo for scanned index */
Index *pBest = 0; /* Best index found so far */
int iRoot = pTab->tnum; /* Root page of scanned b-tree */
/* Check if the query is of one of the following forms:
**
** SELECT min(x) FROM ...
** SELECT max(x) FROM ...
**
** If it is, then ask the code in where.c to attempt to sort results
** as if there was an "ORDER ON x" or "ORDER ON x DESC" clause.
** If where.c is able to produce results sorted in this order, then
** add vdbe code to break out of the processing loop after the
** first iteration (since the first iteration of the loop is
** guaranteed to operate on the row with the minimum or maximum
** value of x, the only row required).
**
** A special flag must be passed to sqlite3WhereBegin() to slightly
** modify behaviour as follows:
**
** + If the query is a "SELECT min(x)", then the loop coded by
** where.c should not iterate over any values with a NULL value
** for x.
**
** + The optimizer code in where.c (the thing that decides which
** index or indices to use) should place a different priority on
** satisfying the 'ORDER BY' clause than it does in other cases.
** Refer to code and comments in where.c for details.
*/
flag = minMaxQuery(p);
if( flag ){
assert( !ExprHasProperty(p->pEList->a[0].pExpr, EP_xIsSelect) );
pMinMax = sqlite3ExprListDup(db, p->pEList->a[0].pExpr->x.pList,0);
pDel = pMinMax;
if( pMinMax && !db->mallocFailed ){
pMinMax->a[0].sortOrder = flag!=WHERE_ORDERBY_MIN ?1:0;
pMinMax->a[0].pExpr->op = TK_COLUMN;
sqlite3CodeVerifySchema(pParse, iDb);
sqlite3TableLock(pParse, iDb, pTab->tnum, 0, pTab->zName);
/* Search for the index that has the least amount of columns. If
** there is such an index, and it has less columns than the table
** does, then we can assume that it consumes less space on disk and
** will therefore be cheaper to scan to determine the query result.
** In this case set iRoot to the root page number of the index b-tree
** and pKeyInfo to the KeyInfo structure required to navigate the
** index.
**
** In practice the KeyInfo structure will not be used. It is only
** passed to keep OP_OpenRead happy.
*/
for(pIdx=pTab->pIndex; pIdx; pIdx=pIdx->pNext){
if( !pBest || pIdx->nColumn<pBest->nColumn ){
pBest = pIdx;
}
}
if( pBest && pBest->nColumn<pTab->nCol ){
iRoot = pBest->tnum;
pKeyInfo = sqlite3IndexKeyinfo(pParse, pBest);
}
/* Open a read-only cursor, execute the OP_Count, close the cursor. */
sqlite3VdbeAddOp3(v, OP_OpenRead, iCsr, iRoot, iDb);
if( pKeyInfo ){
sqlite3VdbeChangeP4(v, -1, (char *)pKeyInfo, P4_KEYINFO_HANDOFF);
}
sqlite3VdbeAddOp2(v, OP_Count, iCsr, sAggInfo.aFunc[0].iMem);
sqlite3VdbeAddOp1(v, OP_Close, iCsr);
}else
#endif /* SQLITE_OMIT_BTREECOUNT */
{
/* Check if the query is of one of the following forms:
**
** SELECT min(x) FROM ...
** SELECT max(x) FROM ...
**
** If it is, then ask the code in where.c to attempt to sort results
** as if there was an "ORDER ON x" or "ORDER ON x DESC" clause.
** If where.c is able to produce results sorted in this order, then
** add vdbe code to break out of the processing loop after the
** first iteration (since the first iteration of the loop is
** guaranteed to operate on the row with the minimum or maximum
** value of x, the only row required).
**
** A special flag must be passed to sqlite3WhereBegin() to slightly
** modify behaviour as follows:
**
** + If the query is a "SELECT min(x)", then the loop coded by
** where.c should not iterate over any values with a NULL value
** for x.
**
** + The optimizer code in where.c (the thing that decides which
** index or indices to use) should place a different priority on
** satisfying the 'ORDER BY' clause than it does in other cases.
** Refer to code and comments in where.c for details.
*/
ExprList *pMinMax = 0;
u8 flag = minMaxQuery(p);
if( flag ){
assert( !ExprHasProperty(p->pEList->a[0].pExpr, EP_xIsSelect) );
pMinMax = sqlite3ExprListDup(db, p->pEList->a[0].pExpr->x.pList,0);
pDel = pMinMax;
if( pMinMax && !db->mallocFailed ){
pMinMax->a[0].sortOrder = flag!=WHERE_ORDERBY_MIN ?1:0;
pMinMax->a[0].pExpr->op = TK_COLUMN;
}
}
/* This case runs if the aggregate has no GROUP BY clause. The
** processing is much simpler since there is only a single row
** of output.
*/
resetAccumulator(pParse, &sAggInfo);
pWInfo = sqlite3WhereBegin(pParse, pTabList, pWhere, &pMinMax, flag, 0);
if( pWInfo==0 ){
sqlite3ExprListDelete(db, pDel);
goto select_end;
}
updateAccumulator(pParse, &sAggInfo);
if( !pMinMax && flag ){
sqlite3VdbeAddOp2(v, OP_Goto, 0, pWInfo->iBreak);
VdbeComment((v, "%s() by index",
(flag==WHERE_ORDERBY_MIN?"min":"max")));
}
sqlite3WhereEnd(pWInfo);
finalizeAggFunctions(pParse, &sAggInfo);
pOrderBy = 0;
if( pHaving ){
sqlite3ExprIfFalse(pParse, pHaving, addrEnd, SQLITE_JUMPIFNULL);
}
}
/* This case runs if the aggregate has no GROUP BY clause. The
** processing is much simpler since there is only a single row
** of output.
*/
resetAccumulator(pParse, &sAggInfo);
pWInfo = sqlite3WhereBegin(pParse, pTabList, pWhere, &pMinMax, flag, 0);
if( pWInfo==0 ){
sqlite3ExprListDelete(db, pDel);
goto select_end;
}
updateAccumulator(pParse, &sAggInfo);
if( !pMinMax && flag ){
sqlite3VdbeAddOp2(v, OP_Goto, 0, pWInfo->iBreak);
VdbeComment((v, "%s() by index",(flag==WHERE_ORDERBY_MIN?"min":"max")));
}
sqlite3WhereEnd(pWInfo);
finalizeAggFunctions(pParse, &sAggInfo);
pOrderBy = 0;
if( pHaving ){
sqlite3ExprIfFalse(pParse, pHaving, addrEnd, SQLITE_JUMPIFNULL);
}
selectInnerLoop(pParse, p, p->pEList, 0, 0, 0, -1,
pDest, addrEnd, addrEnd);
sqlite3ExprListDelete(db, pDel);
}
sqlite3VdbeResolveLabel(v, addrEnd);