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Use a binary search instead of a linear scan when comparing a sample key against data from the sqlite_stat4 table.

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FossilOrigin-Name: e50dc30523210ba12324d5d8379503610f13aa34
This commit is contained in:
dan
2013-08-08 16:17:12 +00:00
parent ad45ed7414
commit 84c309b6ae
3 changed files with 40 additions and 18 deletions
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44
src/where.c
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@@ -2415,25 +2415,47 @@ static int whereKeyStats(
tRowcnt *aStat /* OUT: stats written here */
){
IndexSample *aSample = pIdx->aSample;
int i;
int isEq = 0;
int iCol = pRec->nField-1;
int iCol = pRec->nField-1; /* Index of required stats in anEq[] etc. */
int iMin = 0; /* Smallest sample not yet tested */
int i = pIdx->nSample; /* Smallest sample larger than or equal to pRec */
int iTest; /* Next sample to test */
int res; /* Result of comparison operation */
assert( pIdx->nSample>0 );
assert( pRec->nField>0 && iCol<pIdx->nColumn );
for(i=0; i<pIdx->nSample; i++){
int res = sqlite3VdbeRecordCompare(aSample[i].n, aSample[i].p, pRec);
if( res>=0 ){
isEq = (res==0);
break;
do{
iTest = (iMin+i)/2;
res = sqlite3VdbeRecordCompare(aSample[iTest].n, aSample[iTest].p, pRec);
if( res<0 ){
iMin = iTest+1;
}else{
i = iTest;
}
}while( res && iMin<i );
#ifdef SQLITE_DEBUG
/* The following assert statements check that the binary search code
** above found the right answer. This block serves no purpose other
** than to invoke the asserts. */
if( res==0 ){
/* If (res==0) is true, then sample $i must be equal to pRec */
assert( i<pIdx->nSample );
assert( 0==sqlite3VdbeRecordCompare(aSample[i].n, aSample[i].p, pRec) );
}else{
/* Otherwise, pRec must be smaller than sample $i and larger than
** sample ($i-1). */
assert( i==pIdx->nSample
|| sqlite3VdbeRecordCompare(aSample[i].n, aSample[i].p, pRec)>0 );
assert( i==0
|| sqlite3VdbeRecordCompare(aSample[i-1].n, aSample[i-1].p, pRec)<0 );
}
#endif /* ifdef SQLITE_DEBUG */
/* At this point, aSample[i] is the first sample that is greater than
** or equal to pVal. Or if i==pIdx->nSample, then all samples are less
** than pVal. If aSample[i]==pVal, then isEq==1.
** than pVal. If aSample[i]==pVal, then res==0.
*/
if( isEq ){
assert( i<pIdx->nSample );
if( res==0 ){
aStat[0] = aSample[i].anLt[iCol];
aStat[1] = aSample[i].anEq[iCol];
}else{