database/sqlite
1
0
Fork 0
mirror of https://github.com/sqlite/sqlite.git synced 2025-11-22 20:22:44 +03:00
Code Activity

Attempt to use a covering index even on a full table scan, under the theory

Browse Source 
that the index will be smaller and require less disk I/O and thus be faster.

FossilOrigin-Name: cfaa7bc12847a7006ccc93815f2395ad5259744a
This commit is contained in:
drh
2012-09-15 18:45:54 +00:00
parent 55b4c226d1
commit 3f4d1d1b02
23 changed files with 121 additions and 94 deletions
Download Patch File Download Diff File Expand all files Collapse all files

31
src/where.c
View File

@@ -264,6 +264,7 @@ struct WhereCost {
#define WHERE_MULTI_OR 0x10000000 /* OR using multiple indices */
#define WHERE_TEMP_INDEX 0x20000000 /* Uses an ephemeral index */
#define WHERE_DISTINCT 0x40000000 /* Correct order for DISTINCT */
#define WHERE_COVER_SCAN 0x80000000 /* Full scan of a covering index */
/*
** Initialize a preallocated WhereClause structure.
@@ -3133,7 +3134,7 @@ static void bestBtreeIndex(
** using the main table (i.e. if the index is a covering
** index for this query). If it is, set the WHERE_IDX_ONLY flag in
** wsFlags. Otherwise, set the bLookup variable to true. */
if( pIdx && wsFlags ){
if( pIdx ){
Bitmask m = pSrc->colUsed;
int j;
for(j=0; j<pIdx->nColumn; j++){
@@ -3198,7 +3199,16 @@ static void bestBtreeIndex(
** So this computation assumes table records are about twice as big
** as index records
*/
if( (wsFlags & WHERE_NOT_FULLSCAN)==0 ){
if( wsFlags==WHERE_IDX_ONLY
&& (pWC->wctrlFlags & WHERE_ONEPASS_DESIRED)==0
){
/* This index is not useful for indexing, but it is a covering index.
** A full-scan of the index might be a little faster than a full-scan
** of the table, so give this case a cost slightly less than a table
** scan. */
cost = aiRowEst[0]*3;
wsFlags |= WHERE_COVER_SCAN|WHERE_COLUMN_RANGE;
}else if( (wsFlags & WHERE_NOT_FULLSCAN)==0 ){
/* The cost of a full table scan is a number of move operations equal
** to the number of rows in the table.
**
@@ -4252,6 +4262,11 @@ static Bitmask codeOneLoopStart(
pLevel->op = OP_Next;
}
pLevel->p1 = iIdxCur;
if( pLevel->plan.wsFlags & WHERE_COVER_SCAN ){
pLevel->p5 = SQLITE_STMTSTATUS_FULLSCAN_STEP;
}else{
assert( pLevel->p5==0 );
}
}else
#ifndef SQLITE_OMIT_OR_OPTIMIZATION
@@ -5128,13 +5143,15 @@ WhereInfo *sqlite3WhereBegin(
for(i=0; i<nTabList; i++){
char *z;
int n;
int w;
pLevel = &pWInfo->a[i];
w = pLevel->plan.wsFlags;
pTabItem = &pTabList->a[pLevel->iFrom];
z = pTabItem->zAlias;
if( z==0 ) z = pTabItem->pTab->zName;
n = sqlite3Strlen30(z);
if( n+nQPlan < sizeof(sqlite3_query_plan)-10 ){
if( pLevel->plan.wsFlags & WHERE_IDX_ONLY ){
if( (w & WHERE_IDX_ONLY)!=0 && (w & WHERE_COVER_SCAN)==0 ){
memcpy(&sqlite3_query_plan[nQPlan], "{}", 2);
nQPlan += 2;
}else{
@@ -5143,12 +5160,12 @@ WhereInfo *sqlite3WhereBegin(
}
sqlite3_query_plan[nQPlan++] = ' ';
}
testcase( pLevel->plan.wsFlags & WHERE_ROWID_EQ );
testcase( pLevel->plan.wsFlags & WHERE_ROWID_RANGE );
if( pLevel->plan.wsFlags & (WHERE_ROWID_EQ|WHERE_ROWID_RANGE) ){
testcase( w & WHERE_ROWID_EQ );
testcase( w & WHERE_ROWID_RANGE );
if( w & (WHERE_ROWID_EQ|WHERE_ROWID_RANGE) ){
memcpy(&sqlite3_query_plan[nQPlan], "* ", 2);
nQPlan += 2;
}else if( (pLevel->plan.wsFlags & WHERE_INDEXED)!=0 ){
}else if( (w & WHERE_INDEXED)!=0 && (w & WHERE_COVER_SCAN)==0 ){
n = sqlite3Strlen30(pLevel->plan.u.pIdx->zName);
if( n+nQPlan < sizeof(sqlite3_query_plan)-2 ){
memcpy(&sqlite3_query_plan[nQPlan], pLevel->plan.u.pIdx->zName, n);