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Allow multiple recursive terms in the compound SELECT of a recursive CTE.

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This facilitates writing a query to find find the connected components of
an undirected graph.

FossilOrigin-Name: 5481fa8c79c34f434e99ab633ff3d0942a309a74fb0cf38e3d3617b51d5d21dd
This commit is contained in:
drh
2020-10-19 01:23:48 +00:00
parent ff01ee34c9
commit 340558540e
7 changed files with 176 additions and 44 deletions
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94
src/select.c
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@@ -2340,6 +2340,7 @@ static void generateWithRecursiveQuery(
int nCol = p->pEList->nExpr; /* Number of columns in the recursive table */
Vdbe *v = pParse->pVdbe; /* The prepared statement under construction */
Select *pSetup = p->pPrior; /* The setup query */
Select *pFirstRec; /* Left-most recursive term */
int addrTop; /* Top of the loop */
int addrCont, addrBreak; /* CONTINUE and BREAK addresses */
int iCurrent = 0; /* The Current table */
@@ -2415,7 +2416,25 @@ static void generateWithRecursiveQuery(
/* Detach the ORDER BY clause from the compound SELECT */
p->pOrderBy = 0;
/* Figure out how many elements of the compound SELECT are part of the
** recursive query. Make sure no recursive elements use aggregate
** functions. Mark the recursive elements as UNION ALL even if they
** are really UNION because the distinctness will be enforced by the
** iDistinct table. pFirstRec is left pointing to the left-most
** recursive term of the CTE.
*/
pFirstRec = p;
for(pFirstRec=p; ALWAYS(pFirstRec!=0); pFirstRec=pFirstRec->pPrior){
if( pFirstRec->selFlags & SF_Aggregate ){
sqlite3ErrorMsg(pParse, "recursive aggregate queries not supported");
goto end_of_recursive_query;
}
pFirstRec->op = TK_ALL;
if( (pFirstRec->pPrior->selFlags & SF_Recursive)==0 ) break;
}
/* Store the results of the setup-query in Queue. */
pSetup = pFirstRec->pPrior;
pSetup->pNext = 0;
ExplainQueryPlan((pParse, 1, "SETUP"));
rc = sqlite3Select(pParse, pSetup, &destQueue);
@@ -2448,15 +2467,11 @@ static void generateWithRecursiveQuery(
/* Execute the recursive SELECT taking the single row in Current as
** the value for the recursive-table. Store the results in the Queue.
*/
if( p->selFlags & SF_Aggregate ){
sqlite3ErrorMsg(pParse, "recursive aggregate queries not supported");
}else{
p->pPrior = 0;
ExplainQueryPlan((pParse, 1, "RECURSIVE STEP"));
sqlite3Select(pParse, p, &destQueue);
assert( p->pPrior==0 );
p->pPrior = pSetup;
}
pFirstRec->pPrior = 0;
ExplainQueryPlan((pParse, 1, "RECURSIVE STEP"));
sqlite3Select(pParse, p, &destQueue);
assert( pFirstRec->pPrior==0 );
pFirstRec->pPrior = pSetup;
/* Keep running the loop until the Queue is empty */
sqlite3VdbeGoto(v, addrTop);
@@ -2525,6 +2540,16 @@ static int multiSelectValues(
return rc;
}
/*
** Return true if the SELECT statement which is known to be the recursive
** part of a recursive CTE still has its anchor terms attached. If the
** anchor terms have already been removed, then return false.
*/
static int hasAnchor(Select *p){
while( p && (p->selFlags & SF_Recursive)!=0 ){ p = p->pPrior; }
return p!=0;
}
/*
** This routine is called to process a compound query form from
** two or more separate queries using UNION, UNION ALL, EXCEPT, or
@@ -2610,7 +2635,7 @@ static int multiSelect(
assert( p->pEList->nExpr==pPrior->pEList->nExpr );
#ifndef SQLITE_OMIT_CTE
if( p->selFlags & SF_Recursive ){
if( (p->selFlags & SF_Recursive)!=0 && hasAnchor(p) ){
generateWithRecursiveQuery(pParse, p, &dest);
}else
#endif
@@ -2701,6 +2726,7 @@ static int multiSelect(
findRightmost(p)->selFlags |= SF_UsesEphemeral;
assert( p->pEList );
}
/* Code the SELECT statements to our left
*/
@@ -4794,8 +4820,10 @@ static int withExpand(
ExprList *pEList;
Select *pSel;
Select *pLeft; /* Left-most SELECT statement */
Select *pRecTerm; /* Left-most recursive term */
int bMayRecursive; /* True if compound joined by UNION [ALL] */
With *pSavedWith; /* Initial value of pParse->pWith */
int iRecTab = -1; /* Cursor for recursive table */
/* If pCte->zCteErr is non-NULL at this point, then this is an illegal
** recursive reference to CTE pCte. Leave an error in pParse and return
@@ -4820,44 +4848,48 @@ static int withExpand(
assert( pFrom->pSelect );
/* Check if this is a recursive CTE. */
pSel = pFrom->pSelect;
pRecTerm = pSel = pFrom->pSelect;
bMayRecursive = ( pSel->op==TK_ALL || pSel->op==TK_UNION );
if( bMayRecursive ){
while( bMayRecursive && pRecTerm->op==pSel->op ){
int i;
SrcList *pSrc = pFrom->pSelect->pSrc;
SrcList *pSrc = pRecTerm->pSrc;
assert( pRecTerm->pPrior!=0 );
for(i=0; i<pSrc->nSrc; i++){
struct SrcList_item *pItem = &pSrc->a[i];
if( pItem->zDatabase==0
&& pItem->zName!=0
&& 0==sqlite3StrICmp(pItem->zName, pCte->zName)
){
){
pItem->pTab = pTab;
pItem->fg.isRecursive = 1;
if( pRecTerm->selFlags & SF_Recursive ){
sqlite3ErrorMsg(pParse,
"multiple references to recursive table: %s", pCte->zName
);
return SQLITE_ERROR;
}
pTab->nTabRef++;
pSel->selFlags |= SF_Recursive;
pRecTerm->selFlags |= SF_Recursive;
if( iRecTab<0 ) iRecTab = pParse->nTab++;
pItem->iCursor = iRecTab;
}
}
if( (pRecTerm->selFlags & SF_Recursive)==0 ) break;
pRecTerm = pRecTerm->pPrior;
}
/* Only one recursive reference is permitted. */
if( pTab->nTabRef>2 ){
sqlite3ErrorMsg(
pParse, "multiple references to recursive table: %s", pCte->zName
);
return SQLITE_ERROR;
}
assert( pTab->nTabRef==1 ||
((pSel->selFlags&SF_Recursive) && pTab->nTabRef==2 ));
pCte->zCteErr = "circular reference: %s";
pSavedWith = pParse->pWith;
pParse->pWith = pWith;
if( bMayRecursive ){
Select *pPrior = pSel->pPrior;
assert( pPrior->pWith==0 );
pPrior->pWith = pSel->pWith;
sqlite3WalkSelect(pWalker, pPrior);
pPrior->pWith = 0;
if( pSel->selFlags & SF_Recursive ){
assert( pRecTerm!=0 );
assert( (pRecTerm->selFlags & SF_Recursive)==0 );
assert( pRecTerm->pNext!=0 );
assert( (pRecTerm->pNext->selFlags & SF_Recursive)!=0 );
assert( pRecTerm->pWith==0 );
pRecTerm->pWith = pSel->pWith;
sqlite3WalkSelect(pWalker, pRecTerm);
pRecTerm->pWith = 0;
}else{
sqlite3WalkSelect(pWalker, pSel);
}