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Add code comments for a "table-of-contents" and various milestone mar…
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…ks in

the 1300+ line long sqlite3Select() function, to help improve readiability
and maintainability.  Comment changes only - no functional changes.

FossilOrigin-Name: 00cfbb9fa5136e6a7847da4e4ab30d320ca79c22acda9db2030e558d59b8c744
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drh committed Aug 21, 2024
1 parent 91937a6 commit 4cc86b6
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12 changes: 6 additions & 6 deletions manifest
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C Fix\sa\sminor\stypo\sin\sa\scomment.
D 2024-08-21T10:37:59.175
C Add\scode\scomments\sfor\sa\s"table-of-contents"\sand\svarious\smilestone\smarks\sin\nthe\s1300+\sline\slong\ssqlite3Select()\sfunction,\sto\shelp\simprove\sreadiability\nand\smaintainability.\s\sComment\schanges\sonly\s-\sno\sfunctional\schanges.
D 2024-08-21T12:01:46.057
F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
F LICENSE.md df5091916dbb40e6e9686186587125e1b2ff51f022cc334e886c19a0e9982724
Expand Down Expand Up @@ -764,7 +764,7 @@ F src/printf.c 6a87534ebfb9e5346011191b1f3a7ebc457f5938c7e4feeea478ecf53f6a41b2
F src/random.c 606b00941a1d7dd09c381d3279a058d771f406c5213c9932bbd93d5587be4b9c
F src/resolve.c 9afed5fd7b9111633bdb74a73cdc47324e28e4dc6c27113e3e9aee38fb9422ab
F src/rowset.c 8432130e6c344b3401a8874c3cb49fefe6873fec593294de077afea2dce5ec97
F src/select.c 534eccfb8e3879cc54d93a55ca5118cf72ed9c5337456c217cc0424da33b87a7
F src/select.c 26bd0ce6e8816f8968f50a377dc4255da4b392793e87ae27acd7276b56a447ce
F src/shell.c.in fb7fe0a39473e7a3ad5ad7076ff49d87aa8ed7fdd98a1595345747ac5d78ac2b
F src/sqlite.h.in 1ad9110150773c38ebababbad11b5cb361bcd3997676dec1c91ac5e0416a7b86
F src/sqlite3.rc 5121c9e10c3964d5755191c80dd1180c122fc3a8
Expand Down Expand Up @@ -2210,8 +2210,8 @@ F vsixtest/vsixtest.tcl 6195aba1f12a5e10efc2b8c0009532167be5e301abe5b31385638080
F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
P 0a835d18d9063b17a1d506fd989ebcf7f0d76593978a3bd5981be59618335f21
R 3a629ede216b591a2dde1a6c7535f80d
P 92c80af1129051c9eded8df170730ad6366b4f7715dead34c4364c8149d0dce9
R 96f2b1d6b34bcd20452ccf075e5a2b9a
U drh
Z d8ab38fee62be945af061484336e5eb9
Z 790a1d73ddaf8f8dea50d658062e6edf
# Remove this line to create a well-formed Fossil manifest.
2 changes: 1 addition & 1 deletion manifest.uuid
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92c80af1129051c9eded8df170730ad6366b4f7715dead34c4364c8149d0dce9
00cfbb9fa5136e6a7847da4e4ab30d320ca79c22acda9db2030e558d59b8c744
109 changes: 82 additions & 27 deletions src/select.c
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Original file line number Diff line number Diff line change
Expand Up @@ -7340,7 +7340,7 @@ static int fromClauseTermCanBeCoroutine(
}

/*
** 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.
Expand All @@ -7351,6 +7351,40 @@ static int fromClauseTermCanBeCoroutine(
**
** 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 */
Expand Down Expand Up @@ -7394,6 +7428,7 @@ int sqlite3Select(
}
#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 );
Expand Down Expand Up @@ -7480,6 +7515,7 @@ int sqlite3Select(

/* 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++){
Expand All @@ -7502,6 +7538,7 @@ int sqlite3Select(
** 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,
Expand Down Expand Up @@ -7572,7 +7609,8 @@ int sqlite3Select(
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
Expand Down Expand Up @@ -7638,6 +7676,7 @@ int sqlite3Select(
continue;
}

/* tag-select-0240 */
if( flattenSubquery(pParse, p, i, isAgg) ){
if( pParse->nErr ) goto select_end;
/* This subquery can be absorbed into its parent. */
Expand All @@ -7653,7 +7692,7 @@ int sqlite3Select(

#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);
Expand All @@ -7669,9 +7708,9 @@ int sqlite3Select(
#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
Expand All @@ -7688,16 +7727,20 @@ int sqlite3Select(
TREETRACE(0x2000,pParse,p,("Constant propagation not helpful\n"));
}

/* tag-select-0350 */
if( OptimizationEnabled(db, SQLITE_QueryFlattener|SQLITE_CountOfView)
&& countOfViewOptimization(pParse, p)
){
if( db->mallocFailed ) goto select_end;
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];
Expand All @@ -7709,7 +7752,9 @@ int sqlite3Select(
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:
Expand Down Expand Up @@ -7745,10 +7790,9 @@ int sqlite3Select(
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
Expand All @@ -7761,6 +7805,7 @@ int sqlite3Select(

/* 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
Expand All @@ -7781,6 +7826,7 @@ int sqlite3Select(

/* 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)
Expand All @@ -7798,11 +7844,11 @@ int sqlite3Select(
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;

Expand All @@ -7822,8 +7868,8 @@ int sqlite3Select(
}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 ){
Expand All @@ -7833,7 +7879,7 @@ int sqlite3Select(
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;
Expand All @@ -7847,7 +7893,7 @@ int sqlite3Select(
}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
Expand Down Expand Up @@ -7907,7 +7953,9 @@ int sqlite3Select(
}
#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:
**
Expand Down Expand Up @@ -7957,7 +8005,7 @@ int sqlite3Select(
** 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;
Expand All @@ -7974,6 +8022,7 @@ int sqlite3Select(
}

/* 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);
Expand All @@ -7991,7 +8040,7 @@ int sqlite3Select(
}
}

/* Set the limiter.
/* Set the limiter. tag-select-0650
*/
iEnd = sqlite3VdbeMakeLabel(pParse);
if( (p->selFlags & SF_FixedLimit)==0 ){
Expand All @@ -8003,7 +8052,7 @@ int sqlite3Select(
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++;
Expand All @@ -8018,7 +8067,7 @@ int sqlite3Select(
}

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
Expand Down Expand Up @@ -8091,8 +8140,8 @@ int sqlite3Select(
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 */
Expand Down Expand Up @@ -8211,7 +8260,7 @@ int sqlite3Select(


/* 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 */
Expand Down Expand Up @@ -8508,9 +8557,12 @@ int sqlite3Select(
}
} /* 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>
Expand Down Expand Up @@ -8569,6 +8621,8 @@ int sqlite3Select(
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;
Expand Down Expand Up @@ -8657,7 +8711,7 @@ int sqlite3Select(
}

/* 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 );
Expand All @@ -8680,6 +8734,7 @@ int sqlite3Select(
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 ){
Expand Down

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