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Overview
| Comment: | Improvements to query planning for joins: Avoid unnecessary calls to "optimal scan" checks in cases where table reordering is not possible. Make sure optimal scan checks are carried out for CROSS JOINs and LEFT JOINs. |
|---|---|
| Downloads: | Tarball | ZIP archive | SQL archive |
| Timelines: | family | ancestors | descendants | both | trunk |
| Files: | files | file ages | folders |
| SHA1: |
d5ebb7877885839e93eee3b322624d4c |
| User & Date: | drh 2013-01-16 00:46:09 |
Context
|
2013-01-16
| ||
| 20:33 | Fix the activate_extensions pragma so that it is a no-op when the required argument is omitted. (check-in: 6195ebd8 user: drh tags: trunk) | |
| 17:08 | Enhance the query planner to exploit transitivity of join constraints in a multi-way join. (check-in: 13171eb5 user: drh tags: transitive-constraints) | |
| 00:46 | Improvements to query planning for joins: Avoid unnecessary calls to "optimal scan" checks in cases where table reordering is not possible. Make sure optimal scan checks are carried out for CROSS JOINs and LEFT JOINs. (check-in: d5ebb787 user: drh tags: trunk) | |
|
2013-01-15
| ||
| 18:49 | Fix a missing word in a comment. Enhance the "wheretrace" debugging output to show the estimated cost of each table option while planning the join order. (check-in: ac4e119a user: drh tags: trunk) | |
Changes
Changes to src/where.c.
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for(sWBI.i=iFrom=0, pLevel=pWInfo->a; sWBI.i<nTabList; sWBI.i++, pLevel++){
WhereCost bestPlan; /* Most efficient plan seen so far */
Index *pIdx; /* Index for FROM table at pTabItem */
int j; /* For looping over FROM tables */
int bestJ = -1; /* The value of j */
Bitmask m; /* Bitmask value for j or bestJ */
int isOptimal; /* Iterator for optimal/non-optimal search */
int nUnconstrained; /* Number tables without INDEXED BY */
Bitmask notIndexed; /* Mask of tables that cannot use an index */
memset(&bestPlan, 0, sizeof(bestPlan));
bestPlan.rCost = SQLITE_BIG_DBL;
WHERETRACE(("*** Begin search for loop %d ***\n", sWBI.i));
................................................................................
** Since the cost of a linear scan through table t2 is the same
** as the cost of a linear scan through table t1, a simple greedy
** algorithm may choose to use t2 for the outer loop, which is a much
** costlier approach.
*/
nUnconstrained = 0;
notIndexed = 0;
for(isOptimal=(iFrom<nTabList-1); isOptimal>=0 && bestJ<0; isOptimal--){
for(j=iFrom, sWBI.pSrc=&pTabList->a[j]; j<nTabList; j++, sWBI.pSrc++){
int doNotReorder; /* True if this table should not be reordered */
doNotReorder = (sWBI.pSrc->jointype & (JT_LEFT|JT_CROSS))!=0;
if( j!=iFrom && doNotReorder ) break;
m = getMask(pMaskSet, sWBI.pSrc->iCursor);
if( (m & sWBI.notValid)==0 ){
if( j==iFrom ) iFrom++;
continue;
}
sWBI.notReady = (isOptimal ? m : sWBI.notValid);
if( sWBI.pSrc->pIndex==0 ) nUnconstrained++;
WHERETRACE((" === trying table %d (%s) with isOptimal=%d ===\n",
j, sWBI.pSrc->pTab->zName, isOptimal));
assert( sWBI.pSrc->pTab );
................................................................................
|| sWBI.cost.plan.u.pIdx==sWBI.pSrc->pIndex );
if( isOptimal && (sWBI.cost.plan.wsFlags & WHERE_NOT_FULLSCAN)==0 ){
notIndexed |= m;
}
if( isOptimal ){
pWInfo->a[j].rOptCost = sWBI.cost.rCost;
}else if( iFrom<nTabList-1 ){
/* If two or more tables have nearly the same outer loop cost, but
** very different inner loop (optimal) cost, we want to choose
** for the outer loop that table which benefits the least from
** being in the inner loop. The following code scales the
** outer loop cost estimate to accomplish that. */
WHERETRACE((" scaling cost from %.1f to %.1f\n",
sWBI.cost.rCost,
................................................................................
" cost=%.1f, nRow=%.1f, nOBSat=%d, wsFlags=%08x\n",
j, sWBI.pSrc->pTab->zName,
sWBI.cost.rCost, sWBI.cost.plan.nRow,
sWBI.cost.plan.nOBSat, sWBI.cost.plan.wsFlags));
bestPlan = sWBI.cost;
bestJ = j;
}
if( doNotReorder ) break;
}
}
assert( bestJ>=0 );
assert( sWBI.notValid & getMask(pMaskSet, pTabList->a[bestJ].iCursor) );
WHERETRACE(("*** Optimizer selects table %d (%s) for loop %d with:\n"
" cost=%.1f, nRow=%.1f, nOBSat=%d, wsFlags=0x%08x\n",
bestJ, pTabList->a[bestJ].pTab->zName,
pLevel-pWInfo->a, bestPlan.rCost, bestPlan.plan.nRow,
bestPlan.plan.nOBSat, bestPlan.plan.wsFlags));
if( (bestPlan.plan.wsFlags & WHERE_DISTINCT)!=0 ){
assert( pWInfo->eDistinct==0 );
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5068 5069 5070 5071 5072 5073 5074 5075 5076 5077 5078 5079 5080 5081 5082 .... 5119 5120 5121 5122 5123 5124 5125 5126 5127 5128 5129 5130 5131 5132 5133 5134 5135 5136 5137 5138 5139 5140 5141 5142 5143 5144 5145 5146 5147 5148 5149 5150 5151 5152 5153 5154 5155 5156 5157 5158 5159 5160 5161 5162 5163 5164 5165 5166 .... 5182 5183 5184 5185 5186 5187 5188 5189 5190 5191 5192 5193 5194 5195 5196 .... 5228 5229 5230 5231 5232 5233 5234 5235 5236 5237 5238 5239 5240 5241 5242 5243 5244 5245 5246 5247 5248 5249 5250 5251 5252 5253 5254 |
for(sWBI.i=iFrom=0, pLevel=pWInfo->a; sWBI.i<nTabList; sWBI.i++, pLevel++){
WhereCost bestPlan; /* Most efficient plan seen so far */
Index *pIdx; /* Index for FROM table at pTabItem */
int j; /* For looping over FROM tables */
int bestJ = -1; /* The value of j */
Bitmask m; /* Bitmask value for j or bestJ */
int isOptimal; /* Iterator for optimal/non-optimal search */
int ckOptimal; /* Do the optimal scan check */
int nUnconstrained; /* Number tables without INDEXED BY */
Bitmask notIndexed; /* Mask of tables that cannot use an index */
memset(&bestPlan, 0, sizeof(bestPlan));
bestPlan.rCost = SQLITE_BIG_DBL;
WHERETRACE(("*** Begin search for loop %d ***\n", sWBI.i));
................................................................................
** Since the cost of a linear scan through table t2 is the same
** as the cost of a linear scan through table t1, a simple greedy
** algorithm may choose to use t2 for the outer loop, which is a much
** costlier approach.
*/
nUnconstrained = 0;
notIndexed = 0;
/* The optimal scan check only occurs if there are two or more tables
** available to be reordered */
if( iFrom==nTabList-1 ){
ckOptimal = 0; /* Common case of just one table in the FROM clause */
}else{
ckOptimal = -1;
for(j=iFrom, sWBI.pSrc=&pTabList->a[j]; j<nTabList; j++, sWBI.pSrc++){
m = getMask(pMaskSet, sWBI.pSrc->iCursor);
if( (m & sWBI.notValid)==0 ){
if( j==iFrom ) iFrom++;
continue;
}
if( j>iFrom && (sWBI.pSrc->jointype & (JT_LEFT|JT_CROSS))!=0 ) break;
if( ++ckOptimal ) break;
if( (sWBI.pSrc->jointype & JT_LEFT)!=0 ) break;
}
}
assert( ckOptimal==0 || ckOptimal==1 );
for(isOptimal=ckOptimal; isOptimal>=0 && bestJ<0; isOptimal--){
for(j=iFrom, sWBI.pSrc=&pTabList->a[j]; j<nTabList; j++, sWBI.pSrc++){
if( j>iFrom && (sWBI.pSrc->jointype & (JT_LEFT|JT_CROSS))!=0 ){
/* This break and one like it in the ckOptimal computation loop
** above prevent table reordering across LEFT and CROSS JOINs.
** The LEFT JOIN case is necessary for correctness. The prohibition
** against reordering across a CROSS JOIN is an SQLite feature that
** allows the developer to control table reordering */
break;
}
m = getMask(pMaskSet, sWBI.pSrc->iCursor);
if( (m & sWBI.notValid)==0 ){
assert( j>iFrom );
continue;
}
sWBI.notReady = (isOptimal ? m : sWBI.notValid);
if( sWBI.pSrc->pIndex==0 ) nUnconstrained++;
WHERETRACE((" === trying table %d (%s) with isOptimal=%d ===\n",
j, sWBI.pSrc->pTab->zName, isOptimal));
assert( sWBI.pSrc->pTab );
................................................................................
|| sWBI.cost.plan.u.pIdx==sWBI.pSrc->pIndex );
if( isOptimal && (sWBI.cost.plan.wsFlags & WHERE_NOT_FULLSCAN)==0 ){
notIndexed |= m;
}
if( isOptimal ){
pWInfo->a[j].rOptCost = sWBI.cost.rCost;
}else if( ckOptimal ){
/* If two or more tables have nearly the same outer loop cost, but
** very different inner loop (optimal) cost, we want to choose
** for the outer loop that table which benefits the least from
** being in the inner loop. The following code scales the
** outer loop cost estimate to accomplish that. */
WHERETRACE((" scaling cost from %.1f to %.1f\n",
sWBI.cost.rCost,
................................................................................
" cost=%.1f, nRow=%.1f, nOBSat=%d, wsFlags=%08x\n",
j, sWBI.pSrc->pTab->zName,
sWBI.cost.rCost, sWBI.cost.plan.nRow,
sWBI.cost.plan.nOBSat, sWBI.cost.plan.wsFlags));
bestPlan = sWBI.cost;
bestJ = j;
}
/* In a join like "w JOIN x LEFT JOIN y JOIN z" make sure that
** table y (and not table z) is always the next inner loop inside
** of table x. */
if( (sWBI.pSrc->jointype & JT_LEFT)!=0 ) break;
}
}
assert( bestJ>=0 );
assert( sWBI.notValid & getMask(pMaskSet, pTabList->a[bestJ].iCursor) );
assert( bestJ==iFrom || (pTabList->a[iFrom].jointype & JT_LEFT)==0 );
testcase( bestJ>iFrom && (pTabList->a[iFrom].jointype & JT_CROSS)!=0 );
testcase( bestJ>iFrom && bestJ<nTabList-1
&& (pTabList->a[bestJ+1].jointype & JT_LEFT)!=0 );
WHERETRACE(("*** Optimizer selects table %d (%s) for loop %d with:\n"
" cost=%.1f, nRow=%.1f, nOBSat=%d, wsFlags=0x%08x\n",
bestJ, pTabList->a[bestJ].pTab->zName,
pLevel-pWInfo->a, bestPlan.rCost, bestPlan.plan.nRow,
bestPlan.plan.nOBSat, bestPlan.plan.wsFlags));
if( (bestPlan.plan.wsFlags & WHERE_DISTINCT)!=0 ){
assert( pWInfo->eDistinct==0 );
|