}
/*
-** Generate code for the SELECT statement given in the p argument.
+** Generate byte-code for the SELECT statement given in the p argument.
**
** The results are returned according to the SelectDest structure.
** See comments in sqliteInt.h for further information.
**
** This routine does NOT free the Select structure passed in. The
** calling function needs to do that.
+**
+** This is a long function. The following is an outline of the processing
+** steps, with tags referencing various milestones:
+**
+** * Resolve names and similar preparation tag-select-0100
+** * Scan of the FROM clause tag-select-0200
+** + OUTER JOIN strength reduction tag-select-0220
+** + Sub-query ORDER BY removal tag-select-0230
+** + Query flattening tag-select-0240
+** * Separate subroutine for compound-SELECT tag-select-0300
+** * WHERE-clause constant propagation tag-select-0330
+** * Count()-of-VIEW optimization tag-select-0350
+** * Scan of the FROM clause again tag-select-0400
+** + Authorize unreferenced tables tag-select-0410
+** + Predicate push-down optimization tag-select-0420
+** + Omit unused subquery columns optimization tag-select-0440
+** + Generate code to implement subqueries tag-select-0480
+** - Co-routines tag-select-0482
+** - Reuse previously computed CTE tag-select-0484
+** - REuse previously computed VIEW tag-select-0486
+** - Materialize a VIEW or CTE tag-select-0488
+** * DISTINCT ORDER BY -> GROUP BY optimization tag-select-0500
+** * Set up for ORDER BY tag-select-0600
+** * Create output table tag-select-0630
+** * Prepare registers for LIMIT tag-select-0650
+** * Setup for DISTINCT tag-select-0680
+** * Generate code for non-aggregate and non-GROUP BY tag-select-0700
+** * Generate code for aggregate and/or GROUP BY tag-select-0800
+** + GROUP BY queries tag-select-0810
+** + non-GROUP BY queries tag-select-0820
+** - Special case of count() w/o GROUP BY tag-select-0821
+** - General case of non-GROUP BY aggregates tag-select-0822
+** * Sort results, as needed tag-select-0900
+** * Internal self-checks tag-select-1000
*/
int sqlite3Select(
Parse *pParse, /* The parser context */
}
#endif
+ /* tag-select-0100 */
assert( p->pOrderBy==0 || pDest->eDest!=SRT_DistFifo );
assert( p->pOrderBy==0 || pDest->eDest!=SRT_Fifo );
assert( p->pOrderBy==0 || pDest->eDest!=SRT_DistQueue );
/* Try to do various optimizations (flattening subqueries, and strength
** reduction of join operators) in the FROM clause up into the main query
+ ** tag-select-0200
*/
#if !defined(SQLITE_OMIT_SUBQUERY) || !defined(SQLITE_OMIT_VIEW)
for(i=0; !p->pPrior && i<pTabList->nSrc; i++){
** way that the i-th table cannot be the NULL row of a join, then
** perform the appropriate simplification. This is called
** "OUTER JOIN strength reduction" in the SQLite documentation.
+ ** tag-select-0220
*/
if( (pItem->fg.jointype & (JT_LEFT|JT_LTORJ))!=0
&& sqlite3ExprImpliesNonNullRow(p->pWhere, pItem->iCursor,
if( (pSub->selFlags & SF_Aggregate)!=0 ) continue;
assert( pSub->pGroupBy==0 );
- /* If a FROM-clause subquery has an ORDER BY clause that is not
+ /* tag-select-0230:
+ ** If a FROM-clause subquery has an ORDER BY clause that is not
** really doing anything, then delete it now so that it does not
** interfere with query flattening. See the discussion at
** https://sqlite.org/forum/forumpost/2d76f2bcf65d256a
continue;
}
+ /* tag-select-0240 */
if( flattenSubquery(pParse, p, i, isAgg) ){
if( pParse->nErr ) goto select_end;
/* This subquery can be absorbed into its parent. */
#ifndef SQLITE_OMIT_COMPOUND_SELECT
/* Handle compound SELECT statements using the separate multiSelect()
- ** procedure.
+ ** procedure. tag-select-0300
*/
if( p->pPrior ){
rc = multiSelect(pParse, p, pDest);
#endif
/* Do the WHERE-clause constant propagation optimization if this is
- ** a join. No need to speed time on this operation for non-join queries
+ ** a join. No need to spend time on this operation for non-join queries
** as the equivalent optimization will be handled by query planner in
- ** sqlite3WhereBegin().
+ ** sqlite3WhereBegin(). tag-select-0330
*/
if( p->pWhere!=0
&& p->pWhere->op==TK_AND
TREETRACE(0x2000,pParse,p,("Constant propagation not helpful\n"));
}
+ /* tag-select-0350 */
if( OptimizationEnabled(db, SQLITE_QueryFlattener|SQLITE_CountOfView)
&& countOfViewOptimization(pParse, p)
){
pTabList = p->pSrc;
}
- /* For each term in the FROM clause, do two things:
- ** (1) Authorize unreferenced tables
- ** (2) Generate code for all sub-queries
+ /* Loop over all terms in the FROM clause and do two things for each term:
+ **
+ ** (1) Authorize unreferenced tables
+ ** (2) Generate code for all sub-queries
+ **
+ ** tag-select-0400
*/
for(i=0; i<pTabList->nSrc; i++){
SrcItem *pItem = &pTabList->a[i];
const char *zSavedAuthContext;
#endif
- /* Issue SQLITE_READ authorizations with a fake column name for any
+ /* Authorized unreferenced tables. tag-select-0410
+ **
+ ** Issue SQLITE_READ authorizations with a fake column name for any
** tables that are referenced but from which no values are extracted.
** Examples of where these kinds of null SQLITE_READ authorizations
** would occur:
pSubq = pItem->u4.pSubq;
assert( pSubq!=0 );
pSub = pSubq->pSelect;
- if( pSubq->addrFillSub!=0 ) continue;
/* The code for a subquery should only be generated once. */
- assert( pSubq->addrFillSub==0 );
+ if( pSubq->addrFillSub!=0 ) continue;
/* Increment Parse.nHeight by the height of the largest expression
** tree referred to by this, the parent select. The child select
/* Make copies of constant WHERE-clause terms in the outer query down
** inside the subquery. This can help the subquery to run more efficiently.
+ ** This is the "predicate push-down optimization". tag-select-0420
*/
if( OptimizationEnabled(db, SQLITE_PushDown)
&& (pItem->fg.isCte==0
/* Convert unused result columns of the subquery into simple NULL
** expressions, to avoid unneeded searching and computation.
+ ** tag-select-0440
*/
if( OptimizationEnabled(db, SQLITE_NullUnusedCols)
&& disableUnusedSubqueryResultColumns(pItem)
zSavedAuthContext = pParse->zAuthContext;
pParse->zAuthContext = pItem->zName;
- /* Generate code to implement the subquery
+ /* Generate byte-code to implement the subquery tag-select-0480
*/
if( fromClauseTermCanBeCoroutine(pParse, pTabList, i, p->selFlags) ){
/* Implement a co-routine that will return a single row of the result
- ** set on each invocation.
+ ** set on each invocation. tag-select-0482
*/
int addrTop = sqlite3VdbeCurrentAddr(v)+1;
}else if( pItem->fg.isCte && pItem->u2.pCteUse->addrM9e>0 ){
/* This is a CTE for which materialization code has already been
** generated. Invoke the subroutine to compute the materialization,
- ** the make the pItem->iCursor be a copy of the ephemeral table that
- ** holds the result of the materialization. */
+ ** then make the pItem->iCursor be a copy of the ephemeral table that
+ ** holds the result of the materialization. tag-select-0484 */
CteUse *pCteUse = pItem->u2.pCteUse;
sqlite3VdbeAddOp2(v, OP_Gosub, pCteUse->regRtn, pCteUse->addrM9e);
if( pItem->iCursor!=pCteUse->iCur ){
pSub->nSelectRow = pCteUse->nRowEst;
}else if( (pPrior = isSelfJoinView(pTabList, pItem, 0, i))!=0 ){
/* This view has already been materialized by a prior entry in
- ** this same FROM clause. Reuse it. */
+ ** this same FROM clause. Reuse it. tag-select-0486 */
Subquery *pPriorSubq;
assert( pPrior->fg.isSubquery );
pPriorSubq = pPrior->u4.pSubq;
}else{
/* Materialize the view. If the view is not correlated, generate a
** subroutine to do the materialization so that subsequent uses of
- ** the same view can reuse the materialization. */
+ ** the same view can reuse the materialization. tag-select-0488 */
int topAddr;
int onceAddr = 0;
#ifdef SQLITE_ENABLE_STMT_SCANSTATUS
}
#endif
- /* If the query is DISTINCT with an ORDER BY but is not an aggregate, and
+ /* tag-select-0500
+ **
+ ** If the query is DISTINCT with an ORDER BY but is not an aggregate, and
** if the select-list is the same as the ORDER BY list, then this query
** can be rewritten as a GROUP BY. In other words, this:
**
** If that is the case, then the OP_OpenEphemeral instruction will be
** changed to an OP_Noop once we figure out that the sorting index is
** not needed. The sSort.addrSortIndex variable is used to facilitate
- ** that change.
+ ** that change. tag-select-0600
*/
if( sSort.pOrderBy ){
KeyInfo *pKeyInfo;
}
/* If the output is destined for a temporary table, open that table.
+ ** tag-select-0630
*/
if( pDest->eDest==SRT_EphemTab ){
sqlite3VdbeAddOp2(v, OP_OpenEphemeral, pDest->iSDParm, pEList->nExpr);
}
}
- /* Set the limiter.
+ /* Set the limiter. tag-select-0650
*/
iEnd = sqlite3VdbeMakeLabel(pParse);
if( (p->selFlags & SF_FixedLimit)==0 ){
sSort.sortFlags |= SORTFLAG_UseSorter;
}
- /* Open an ephemeral index to use for the distinct set.
+ /* Open an ephemeral index to use for the distinct set. tag-select-0680
*/
if( p->selFlags & SF_Distinct ){
sDistinct.tabTnct = pParse->nTab++;
}
if( !isAgg && pGroupBy==0 ){
- /* No aggregate functions and no GROUP BY clause */
+ /* No aggregate functions and no GROUP BY clause. tag-select-0700 */
u16 wctrlFlags = (sDistinct.isTnct ? WHERE_WANT_DISTINCT : 0)
| (p->selFlags & SF_FixedLimit);
#ifndef SQLITE_OMIT_WINDOWFUNC
sqlite3WhereEnd(pWInfo);
}
}else{
- /* This case when there exist aggregate functions or a GROUP BY clause
- ** or both */
+ /* This case is for when there exist aggregate functions or a GROUP BY
+ ** clause or both. tag-select-0800 */
NameContext sNC; /* Name context for processing aggregate information */
int iAMem; /* First Mem address for storing current GROUP BY */
int iBMem; /* First Mem address for previous GROUP BY */
/* Processing for aggregates with GROUP BY is very different and
- ** much more complex than aggregates without a GROUP BY.
+ ** much more complex than aggregates without a GROUP BY. tag-select-0810
*/
if( pGroupBy ){
KeyInfo *pKeyInfo; /* Keying information for the group by clause */
}
} /* endif pGroupBy. Begin aggregate queries without GROUP BY: */
else {
+ /* Aggregate functions without GROUP BY. tag-select-0820 */
Table *pTab;
if( (pTab = isSimpleCount(p, pAggInfo))!=0 ){
- /* If isSimpleCount() returns a pointer to a Table structure, then
+ /* tag-select-0821
+ **
+ ** If isSimpleCount() returns a pointer to a Table structure, then
** the SQL statement is of the form:
**
** SELECT count(*) FROM <tbl>
sqlite3VdbeAddOp1(v, OP_Close, iCsr);
explainSimpleCount(pParse, pTab, pBest);
}else{
+ /* The general case of an aggregate query without GROUP BY
+ ** tag-select-0822 */
int regAcc = 0; /* "populate accumulators" flag */
ExprList *pDistinct = 0;
u16 distFlag = 0;
}
/* If there is an ORDER BY clause, then we need to sort the results
- ** and send them to the callback one by one.
+ ** and send them to the callback one by one. tag-select-0900
*/
if( sSort.pOrderBy ){
assert( p->pEList==pEList );
assert( db->mallocFailed==0 || pParse->nErr!=0 );
sqlite3ExprListDelete(db, pMinMaxOrderBy);
#ifdef SQLITE_DEBUG
+ /* Internal self-checks. tag-select-1000 */
if( pAggInfo && !db->mallocFailed ){
#if TREETRACE_ENABLED
if( sqlite3TreeTrace & 0x20 ){
projects / thirdparty / sqlite.git / commitdiff
