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Change the recursive common table expression algorithm to use a queue instead

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of a pair of tables.  Runs about 25% faster on the sudoku solver query. 
The OP_SwapCursors opcode is no longer required.  The current implementation
uses just a fifo, but the plan is to change it into a queue that will support 
ORDER BY and LIMIT in a recursive query.

FossilOrigin-Name: b2671e1133d2f1fbd36e7cd4b86d6cc7b528aa97
This commit is contained in:
drh
2014-01-21 22:25:45 +00:00
parent 8561c81ed6
commit e73f059093
6 changed files with 76 additions and 86 deletions
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91
src/select.c
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@@ -1792,20 +1792,21 @@ static int multiSelect(
#ifndef SQLITE_OMIT_CTE
if( p->selFlags & SF_Recursive ){
SrcList *pSrc = p->pSrc;
int nCol = p->pEList->nExpr;
int addrNext;
int addrSwap;
int iCont, iBreak;
int tmp1; /* Intermediate table */
int tmp2; /* Next intermediate table */
int tmp3 = 0; /* To ensure unique results if UNION */
int eDest = SRT_Table;
SelectDest tmp2dest;
int i;
SrcList *pSrc = p->pSrc; /* The FROM clause of the recursive query */
int nCol = p->pEList->nExpr; /* Number of columns in the CTE */
int addrTop; /* Top of the loop */
int addrCont, addrBreak; /* CONTINUE and BREAK addresses */
int iCurrent; /* The Current table */
int regCurrent; /* Register holding Current table */
int iQueue; /* The Queue table */
int iDistinct; /* To ensure unique results if UNION */
int eDest; /* How to write to Queue */
SelectDest destQueue; /* SelectDest targetting the Queue table */
int i; /* Loop counter */
/* Check that there is no ORDER BY or LIMIT clause. Neither of these
** are supported on recursive queries. */
** are currently supported on recursive queries.
*/
assert( p->pOffset==0 || p->pLimit );
if( p->pOrderBy || p->pLimit ){
sqlite3ErrorMsg(pParse, "%s in a recursive query",
@@ -1817,56 +1818,66 @@ static int multiSelect(
if( sqlite3AuthCheck(pParse, SQLITE_RECURSIVE, 0, 0, 0) ){
goto multi_select_end;
}
iBreak = sqlite3VdbeMakeLabel(v);
iCont = sqlite3VdbeMakeLabel(v);
addrBreak = sqlite3VdbeMakeLabel(v);
addrCont = sqlite3VdbeMakeLabel(v);
/* Locate the cursor number of the Current table */
for(i=0; ALWAYS(i<pSrc->nSrc); i++){
if( pSrc->a[i].isRecursive ){
tmp1 = pSrc->a[i].iCursor;
iCurrent = pSrc->a[i].iCursor;
break;
}
}
tmp2 = pParse->nTab++;
/* Allocate cursors for Queue and Distinct. The cursor number for
** the Distinct table must be exactly one greater than Queue in order
** for the SRT_DistTable destination to work. */
iQueue = pParse->nTab++;
if( p->op==TK_UNION ){
eDest = SRT_DistTable;
tmp3 = pParse->nTab++;
iDistinct = pParse->nTab++;
}else{
eDest = SRT_Table;
iDistinct = 0;
}
sqlite3SelectDestInit(&tmp2dest, eDest, tmp2);
sqlite3SelectDestInit(&destQueue, eDest, iQueue);
sqlite3VdbeAddOp2(v, OP_OpenEphemeral, tmp1, nCol);
sqlite3VdbeAddOp2(v, OP_OpenEphemeral, tmp2, nCol);
if( tmp3 ){
p->addrOpenEphm[0] = sqlite3VdbeAddOp2(v, OP_OpenEphemeral, tmp3, 0);
/* Allocate cursors for Current, Queue, and iDistinct. */
regCurrent = ++pParse->nMem;
sqlite3VdbeAddOp3(v, OP_OpenPseudo, iCurrent, regCurrent, nCol);
sqlite3VdbeAddOp2(v, OP_OpenEphemeral, iQueue, nCol);
if( iDistinct ){
p->addrOpenEphm[0] = sqlite3VdbeAddOp2(v, OP_OpenEphemeral, iDistinct, 0);
p->selFlags |= SF_UsesEphemeral;
}
/* Store the results of the initial SELECT in tmp2. */
rc = sqlite3Select(pParse, pPrior, &tmp2dest);
/* Store the results of the initial SELECT in Queue. */
rc = sqlite3Select(pParse, pPrior, &destQueue);
if( rc ) goto multi_select_end;
/* Clear tmp1. Then switch the contents of tmp1 and tmp2. Then return
** the contents of tmp1 to the caller. Or, if tmp1 is empty at this
** point, the recursive query has finished - jump to address iBreak. */
addrSwap = sqlite3VdbeAddOp2(v, OP_SwapCursors, tmp1, tmp2);
sqlite3VdbeAddOp2(v, OP_Rewind, tmp1, iBreak);
addrNext = sqlite3VdbeCurrentAddr(v);
selectInnerLoop(pParse, p, p->pEList, tmp1, p->pEList->nExpr,
0, 0, &dest, iCont, iBreak);
sqlite3VdbeResolveLabel(v, iCont);
sqlite3VdbeAddOp2(v, OP_Next, tmp1, addrNext);
/* Find the next row in the Queue and output that row */
addrTop = sqlite3VdbeAddOp2(v, OP_Rewind, iQueue, addrBreak);
selectInnerLoop(pParse, p, p->pEList, iQueue, p->pEList->nExpr,
0, 0, &dest, addrCont, addrBreak);
sqlite3VdbeResolveLabel(v, addrCont);
/* Execute the recursive SELECT. Store the results in tmp2. While this
** SELECT is running, the contents of tmp1 are read by recursive
** references to the current CTE. */
/* Transfer the next row in Queue over to Current */
sqlite3VdbeAddOp1(v, OP_NullRow, iCurrent); /* To reset column cache */
sqlite3VdbeAddOp2(v, OP_RowData, iQueue, regCurrent);
sqlite3VdbeAddOp1(v, OP_Delete, iQueue);
/* Execute the recursive SELECT taking the single row in Current as
** the value for the CTE. Store the results in the Queue.
*/
p->pPrior = 0;
rc = sqlite3Select(pParse, p, &tmp2dest);
rc = sqlite3Select(pParse, p, &destQueue);
assert( p->pPrior==0 );
p->pPrior = pPrior;
if( rc ) goto multi_select_end;
sqlite3VdbeAddOp2(v, OP_Goto, 0, addrSwap);
sqlite3VdbeResolveLabel(v, iBreak);
/* Keep running the loop until the Queue is empty */
sqlite3VdbeAddOp2(v, OP_Goto, 0, addrTop);
sqlite3VdbeResolveLabel(v, addrBreak);
}else
#endif