DBD-SQLite2
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/*
** 2001 September 15
**
** The author disclaims copyright to this source code. In place of
** a legal notice, here is a blessing:
**
** May you do good and not evil.
** May you find forgiveness for yourself and forgive others.
** May you share freely, never taking more than you give.
**
*************************************************************************
** This module contains C code that generates VDBE code used to process
** the WHERE clause of SQL statements.
**
** $Id: where.c,v 1.1.1.1 2004/08/08 15:03:58 matt Exp $
*/
#include "sqliteInt.h"
/*
** The query generator uses an array of instances of this structure to
** help it analyze the subexpressions of the WHERE clause. Each WHERE
** clause subexpression is separated from the others by an AND operator.
*/
typedef struct ExprInfo ExprInfo;
struct ExprInfo {
Expr *p; /* Pointer to the subexpression */
u8 indexable; /* True if this subexprssion is usable by an index */
short int idxLeft; /* p->pLeft is a column in this table number. -1 if
** p->pLeft is not the column of any table */
short int idxRight; /* p->pRight is a column in this table number. -1 if
** p->pRight is not the column of any table */
unsigned prereqLeft; /* Bitmask of tables referenced by p->pLeft */
unsigned prereqRight; /* Bitmask of tables referenced by p->pRight */
unsigned prereqAll; /* Bitmask of tables referenced by p */
};
/*
** An instance of the following structure keeps track of a mapping
** between VDBE cursor numbers and bitmasks. The VDBE cursor numbers
** are small integers contained in SrcList_item.iCursor and Expr.iTable
** fields. For any given WHERE clause, we want to track which cursors
** are being used, so we assign a single bit in a 32-bit word to track
** that cursor. Then a 32-bit integer is able to show the set of all
** cursors being used.
*/
typedef struct ExprMaskSet ExprMaskSet;
struct ExprMaskSet {
int n; /* Number of assigned cursor values */
int ix[31]; /* Cursor assigned to each bit */
};
/*
** Determine the number of elements in an array.
*/
#define ARRAYSIZE(X) (sizeof(X)/sizeof(X[0]))
/*
** This routine is used to divide the WHERE expression into subexpressions
** separated by the AND operator.
**
** aSlot[] is an array of subexpressions structures.
** There are nSlot spaces left in this array. This routine attempts to
** split pExpr into subexpressions and fills aSlot[] with those subexpressions.
** The return value is the number of slots filled.
*/
static int exprSplit(int nSlot, ExprInfo *aSlot, Expr *pExpr){
int cnt = 0;
if( pExpr==0 || nSlot<1 ) return 0;
if( nSlot==1 || pExpr->op!=TK_AND ){
aSlot[0].p = pExpr;
return 1;
}
if( pExpr->pLeft->op!=TK_AND ){
aSlot[0].p = pExpr->pLeft;
cnt = 1 + exprSplit(nSlot-1, &aSlot[1], pExpr->pRight);
}else{
cnt = exprSplit(nSlot, aSlot, pExpr->pLeft);
cnt += exprSplit(nSlot-cnt, &aSlot[cnt], pExpr->pRight);
}
return cnt;
}
/*
** Initialize an expression mask set
*/
#define initMaskSet(P) memset(P, 0, sizeof(*P))
/*
** Return the bitmask for the given cursor. Assign a new bitmask
** if this is the first time the cursor has been seen.
*/
static int getMask(ExprMaskSet *pMaskSet, int iCursor){
int i;
for(i=0; i<pMaskSet->n; i++){
if( pMaskSet->ix[i]==iCursor ) return 1<<i;
}
if( i==pMaskSet->n && i<ARRAYSIZE(pMaskSet->ix) ){
pMaskSet->n++;
pMaskSet->ix[i] = iCursor;
return 1<<i;
}
return 0;
}
/*
** Destroy an expression mask set
*/
#define freeMaskSet(P) /* NO-OP */
/*
** This routine walks (recursively) an expression tree and generates
** a bitmask indicating which tables are used in that expression
** tree.
**
** In order for this routine to work, the calling function must have
** previously invoked sqliteExprResolveIds() on the expression. See
** the header comment on that routine for additional information.
** The sqliteExprResolveIds() routines looks for column names and
** sets their opcodes to TK_COLUMN and their Expr.iTable fields to
** the VDBE cursor number of the table.
*/
static int exprTableUsage(ExprMaskSet *pMaskSet, Expr *p){
unsigned int mask = 0;
if( p==0 ) return 0;
if( p->op==TK_COLUMN ){
mask = getMask(pMaskSet, p->iTable);
if( mask==0 ) mask = -1;
return mask;
}
if( p->pRight ){
mask = exprTableUsage(pMaskSet, p->pRight);
}
if( p->pLeft ){
mask |= exprTableUsage(pMaskSet, p->pLeft);
}
if( p->pList ){
int i;
for(i=0; i<p->pList->nExpr; i++){
mask |= exprTableUsage(pMaskSet, p->pList->a[i].pExpr);
}
}
return mask;
}
/*
** Return TRUE if the given operator is one of the operators that is
** allowed for an indexable WHERE clause. The allowed operators are
** "=", "<", ">", "<=", ">=", and "IN".
*/
static int allowedOp(int op){
switch( op ){
case TK_LT:
case TK_LE:
case TK_GT:
case TK_GE:
case TK_EQ:
case TK_IN:
return 1;
default:
return 0;
}
}
/*
** The input to this routine is an ExprInfo structure with only the
** "p" field filled in. The job of this routine is to analyze the
** subexpression and populate all the other fields of the ExprInfo
** structure.
*/
static void exprAnalyze(ExprMaskSet *pMaskSet, ExprInfo *pInfo){
Expr *pExpr = pInfo->p;
pInfo->prereqLeft = exprTableUsage(pMaskSet, pExpr->pLeft);
pInfo->prereqRight = exprTableUsage(pMaskSet, pExpr->pRight);
pInfo->prereqAll = exprTableUsage(pMaskSet, pExpr);
pInfo->indexable = 0;
pInfo->idxLeft = -1;
pInfo->idxRight = -1;
if( allowedOp(pExpr->op) && (pInfo->prereqRight & pInfo->prereqLeft)==0 ){
if( pExpr->pRight && pExpr->pRight->op==TK_COLUMN ){
pInfo->idxRight = pExpr->pRight->iTable;
pInfo->indexable = 1;
}
if( pExpr->pLeft->op==TK_COLUMN ){
pInfo->idxLeft = pExpr->pLeft->iTable;
pInfo->indexable = 1;
}
}
}
/*
** pOrderBy is an ORDER BY clause from a SELECT statement. pTab is the
** left-most table in the FROM clause of that same SELECT statement and
** the table has a cursor number of "base".
**
** This routine attempts to find an index for pTab that generates the
** correct record sequence for the given ORDER BY clause. The return value
** is a pointer to an index that does the job. NULL is returned if the
** table has no index that will generate the correct sort order.
**
** If there are two or more indices that generate the correct sort order
** and pPreferredIdx is one of those indices, then return pPreferredIdx.
**
** nEqCol is the number of columns of pPreferredIdx that are used as
** equality constraints. Any index returned must have exactly this same
** set of columns. The ORDER BY clause only matches index columns beyond the
** the first nEqCol columns.
**
** All terms of the ORDER BY clause must be either ASC or DESC. The
** *pbRev value is set to 1 if the ORDER BY clause is all DESC and it is
** set to 0 if the ORDER BY clause is all ASC.
*/
static Index *findSortingIndex(
Table *pTab, /* The table to be sorted */
int base, /* Cursor number for pTab */
ExprList *pOrderBy, /* The ORDER BY clause */
Index *pPreferredIdx, /* Use this index, if possible and not NULL */
int nEqCol, /* Number of index columns used with == constraints */
int *pbRev /* Set to 1 if ORDER BY is DESC */
){
int i, j;
Index *pMatch;
Index *pIdx;
int sortOrder;
assert( pOrderBy!=0 );
assert( pOrderBy->nExpr>0 );
sortOrder = pOrderBy->a[0].sortOrder & SQLITE_SO_DIRMASK;
for(i=0; i<pOrderBy->nExpr; i++){
Expr *p;
if( (pOrderBy->a[i].sortOrder & SQLITE_SO_DIRMASK)!=sortOrder ){
/* Indices can only be used if all ORDER BY terms are either
** DESC or ASC. Indices cannot be used on a mixture. */
return 0;
}
if( (pOrderBy->a[i].sortOrder & SQLITE_SO_TYPEMASK)!=SQLITE_SO_UNK ){
/* Do not sort by index if there is a COLLATE clause */
return 0;
}
p = pOrderBy->a[i].pExpr;
if( p->op!=TK_COLUMN || p->iTable!=base ){
/* Can not use an index sort on anything that is not a column in the
** left-most table of the FROM clause */
return 0;
}
}
/* If we get this far, it means the ORDER BY clause consists only of
** ascending columns in the left-most table of the FROM clause. Now
** check for a matching index.
*/
pMatch = 0;
for(pIdx=pTab->pIndex; pIdx; pIdx=pIdx->pNext){
int nExpr = pOrderBy->nExpr;
*pbRev = sortOrder==SQLITE_SO_DESC;
}
return pMatch;
}
/*
** Disable a term in the WHERE clause. Except, do not disable the term
** if it controls a LEFT OUTER JOIN and it did not originate in the ON
** or USING clause of that join.
**
** Consider the term t2.z='ok' in the following queries:
**
** (1) SELECT * FROM t1 LEFT JOIN t2 ON t1.a=t2.x WHERE t2.z='ok'
** (2) SELECT * FROM t1 LEFT JOIN t2 ON t1.a=t2.x AND t2.z='ok'
** (3) SELECT * FROM t1, t2 WHERE t1.a=t2.x AND t2.z='ok'
**
** The t2.z='ok' is disabled in the in (2) because it did not originate
** in the ON clause. The term is disabled in (3) because it is not part
** of a LEFT OUTER JOIN. In (1), the term is not disabled.
**
** Disabling a term causes that term to not be tested in the inner loop
** of the join. Disabling is an optimization. We would get the correct
** results if nothing were ever disabled, but joins might run a little
** slower. The trick is to disable as much as we can without disabling
** too much. If we disabled in (1), we'd get the wrong answer.
** See ticket #813.
*/
static void disableTerm(WhereLevel *pLevel, Expr **ppExpr){
Expr *pExpr = *ppExpr;
if( pLevel->iLeftJoin==0 || ExprHasProperty(pExpr, EP_FromJoin) ){
*ppExpr = 0;
}
}
/*
** Generate the beginning of the loop used for WHERE clause processing.
** The return value is a pointer to an (opaque) structure that contains
** information needed to terminate the loop. Later, the calling routine
** should invoke sqliteWhereEnd() with the return value of this function
** in order to complete the WHERE clause processing.
**
** If an error occurs, this routine returns NULL.
**
** The basic idea is to do a nested loop, one loop for each table in
** the FROM clause of a select. (INSERT and UPDATE statements are the
** same as a SELECT with only a single table in the FROM clause.) For
** example, if the SQL is this:
**
** SELECT * FROM t1, t2, t3 WHERE ...;
**
** Then the code generated is conceptually like the following:
**
** foreach row1 in t1 do \ Code generated
** foreach row2 in t2 do |-- by sqliteWhereBegin()
** foreach row3 in t3 do /
** ...
** end \ Code generated
** end |-- by sqliteWhereEnd()
** end /
**
** There are Btree cursors associated with each table. t1 uses cursor
** number pTabList->a[0].iCursor. t2 uses the cursor pTabList->a[1].iCursor.
** And so forth. This routine generates code to open those VDBE cursors
** and sqliteWhereEnd() generates the code to close them.
**
** If the WHERE clause is empty, the foreach loops must each scan their
** entire tables. Thus a three-way join is an O(N^3) operation. But if
** the tables have indices and there are terms in the WHERE clause that
** refer to those indices, a complete table scan can be avoided and the
** code will run much faster. Most of the work of this routine is checking
** to see if there are indices that can be used to speed up the loop.
**
** Terms of the WHERE clause are also used to limit which rows actually
** make it to the "..." in the middle of the loop. After each "foreach",
** terms of the WHERE clause that use only terms in that loop and outer
** loops are evaluated and if false a jump is made around all subsequent
** inner loops (or around the "..." if the test occurs within the inner-
** most loop)
**
** OUTER JOINS
**
** An outer join of tables t1 and t2 is conceptally coded as follows:
**
** foreach row1 in t1 do
** flag = 0
** foreach row2 in t2 do
** start:
** ...
** flag = 1
** end
** if flag==0 then
** move the row2 cursor to a null row
** goto start
** fi
** end
**
** ORDER BY CLAUSE PROCESSING
**
** *ppOrderBy is a pointer to the ORDER BY clause of a SELECT statement,
** if there is one. If there is no ORDER BY clause or if this routine
** is called from an UPDATE or DELETE statement, then ppOrderBy is NULL.
**
** If an index can be used so that the natural output order of the table
** scan is correct for the ORDER BY clause, then that index is used and
** *ppOrderBy is set to NULL. This is an optimization that prevents an
** unnecessary sort of the result set if an index appropriate for the
** ORDER BY clause already exists.
**
** If the where clause loops cannot be arranged to provide the correct
** output order, then the *ppOrderBy is unchanged.
*/
WhereInfo *sqliteWhereBegin(
Parse *pParse, /* The parser context */
SrcList *pTabList, /* A list of all tables to be scanned */
Expr *pWhere, /* The WHERE clause */
int pushKey, /* If TRUE, leave the table key on the stack */
ExprList **ppOrderBy /* An ORDER BY clause, or NULL */
){
int i; /* Loop counter */
WhereInfo *pWInfo; /* Will become the return value of this function */
Vdbe *v = pParse->pVdbe; /* The virtual database engine */
int brk, cont = 0; /* Addresses used during code generation */
int nExpr; /* Number of subexpressions in the WHERE clause */
int loopMask; /* One bit set for each outer loop */
int haveKey; /* True if KEY is on the stack */
ExprMaskSet maskSet; /* The expression mask set */
int iDirectEq[32]; /* Term of the form ROWID==X for the N-th table */
int iDirectLt[32]; /* Term of the form ROWID<X or ROWID<=X */
int iDirectGt[32]; /* Term of the form ROWID>X or ROWID>=X */
ExprInfo aExpr[101]; /* The WHERE clause is divided into these expressions */
/* pushKey is only allowed if there is a single table (as in an INSERT or
** UPDATE statement)
*/
assert( pushKey==0 || pTabList->nSrc==1 );
/* Split the WHERE clause into separate subexpressions where each
** subexpression is separated by an AND operator. If the aExpr[]
** array fills up, the last entry might point to an expression which
** contains additional unfactored AND operators.
*/
initMaskSet(&maskSet);
memset(aExpr, 0, sizeof(aExpr));
nExpr = exprSplit(ARRAYSIZE(aExpr), aExpr, pWhere);
if( nExpr==ARRAYSIZE(aExpr) ){
sqliteErrorMsg(pParse, "WHERE clause too complex - no more "
"than %d terms allowed", (int)ARRAYSIZE(aExpr)-1);
return 0;
}
/* Allocate and initialize the WhereInfo structure that will become the
** return value.
sqliteFree(pWInfo);
return 0;
}
pWInfo->pParse = pParse;
pWInfo->pTabList = pTabList;
pWInfo->peakNTab = pWInfo->savedNTab = pParse->nTab;
pWInfo->iBreak = sqliteVdbeMakeLabel(v);
/* Special case: a WHERE clause that is constant. Evaluate the
** expression and either jump over all of the code or fall thru.
*/
if( pWhere && (pTabList->nSrc==0 || sqliteExprIsConstant(pWhere)) ){
sqliteExprIfFalse(pParse, pWhere, pWInfo->iBreak, 1);
pWhere = 0;
}
/* Analyze all of the subexpressions.
*/
for(i=0; i<nExpr; i++){
exprAnalyze(&maskSet, &aExpr[i]);
/* If we are executing a trigger body, remove all references to
** new.* and old.* tables from the prerequisite masks.
*/
if( pParse->trigStack ){
int x;
if( (x = pParse->trigStack->newIdx) >= 0 ){
int mask = ~getMask(&maskSet, x);
aExpr[i].prereqRight &= mask;
aExpr[i].prereqLeft &= mask;
aExpr[i].prereqAll &= mask;
}
if( (x = pParse->trigStack->oldIdx) >= 0 ){
int mask = ~getMask(&maskSet, x);
aExpr[i].prereqRight &= mask;
aExpr[i].prereqLeft &= mask;
aExpr[i].prereqAll &= mask;
}
}
}
/* Figure out what index to use (if any) for each nested loop.
** Make pWInfo->a[i].pIdx point to the index to use for the i-th nested
** loop where i==0 is the outer loop and i==pTabList->nSrc-1 is the inner
** loop.
**
** If terms exist that use the ROWID of any table, then set the
** iDirectEq[], iDirectLt[], or iDirectGt[] elements for that table
** to the index of the term containing the ROWID. We always prefer
** to use a ROWID which can directly access a table rather than an
** index which requires reading an index first to get the rowid then
** doing a second read of the actual database table.
**
** Actually, if there are more than 32 tables in the join, only the
** first 32 tables are candidates for indices. This is (again) due
** to the limit of 32 bits in an integer bitmask.
*/
loopMask = 0;
for(i=0; i<pTabList->nSrc && i<ARRAYSIZE(iDirectEq); i++){
int j;
int iCur = pTabList->a[i].iCursor; /* The cursor for this table */
int mask = getMask(&maskSet, iCur); /* Cursor mask for this table */
Table *pTab = pTabList->a[i].pTab;
Index *pIdx;
Index *pBestIdx = 0;
int bestScore = 0;
/* Check to see if there is an expression that uses only the
** ROWID field of this table. For terms of the form ROWID==expr
** set iDirectEq[i] to the index of the term. For terms of the
** form ROWID<expr or ROWID<=expr set iDirectLt[i] to the term index.
** For terms like ROWID>expr or ROWID>=expr set iDirectGt[i].
**
** (Added:) Treat ROWID IN expr like ROWID=expr.
*/
pWInfo->a[i].iCur = -1;
iDirectEq[i] = -1;
iDirectLt[i] = -1;
iDirectGt[i] = -1;
for(j=0; j<nExpr; j++){
if( aExpr[j].idxLeft==iCur && aExpr[j].p->pLeft->iColumn<0
&& (aExpr[j].prereqRight & loopMask)==aExpr[j].prereqRight ){
switch( aExpr[j].p->op ){
case TK_IN:
case TK_EQ: iDirectEq[i] = j; break;
case TK_LE:
case TK_LT: iDirectLt[i] = j; break;
case TK_GE:
case TK_GT: iDirectGt[i] = j; break;
}
}
if( aExpr[j].idxRight==iCur && aExpr[j].p->pRight->iColumn<0
&& (aExpr[j].prereqLeft & loopMask)==aExpr[j].prereqLeft ){
switch( aExpr[j].p->op ){
case TK_EQ: iDirectEq[i] = j; break;
case TK_LE:
case TK_LT: iDirectGt[i] = j; break;
case TK_GE:
case TK_GT: iDirectLt[i] = j; break;
}
}
}
if( iDirectEq[i]>=0 ){
loopMask |= mask;
pWInfo->a[i].pIdx = 0;
continue;
}
/* Do a search for usable indices. Leave pBestIdx pointing to
** the "best" index. pBestIdx is left set to NULL if no indices
** are usable.
**
** The best index is determined as follows. For each of the
** left-most terms that is fixed by an equality operator, add
** 8 to the score. The right-most term of the index may be
** constrained by an inequality. Add 1 if for an "x<..." constraint
** and add 2 for an "x>..." constraint. Chose the index that
** gives the best score.
**
** This scoring system is designed so that the score can later be
** used to determine how the index is used. If the score&7 is 0
** no outer joins with DELETE and UPDATE.
*/
haveKey = 0;
sqliteVdbeAddOp(v, OP_MoveTo, iCur, 0);
}
sqliteExprIfFalse(pParse, aExpr[j].p, cont, 1);
aExpr[j].p = 0;
}
}
}
pWInfo->iContinue = cont;
if( pushKey && !haveKey ){
sqliteVdbeAddOp(v, OP_Recno, pTabList->a[0].iCursor, 0);
}
freeMaskSet(&maskSet);
return pWInfo;
}
/*
** Generate the end of the WHERE loop. See comments on
** sqliteWhereBegin() for additional information.
*/
void sqliteWhereEnd(WhereInfo *pWInfo){
Vdbe *v = pWInfo->pParse->pVdbe;
int i;
WhereLevel *pLevel;
SrcList *pTabList = pWInfo->pTabList;
for(i=pTabList->nSrc-1; i>=0; i--){
pLevel = &pWInfo->a[i];
sqliteVdbeResolveLabel(v, pLevel->cont);
if( pLevel->op!=OP_Noop ){
sqliteVdbeAddOp(v, pLevel->op, pLevel->p1, pLevel->p2);
}
sqliteVdbeResolveLabel(v, pLevel->brk);
if( pLevel->inOp!=OP_Noop ){
sqliteVdbeAddOp(v, pLevel->inOp, pLevel->inP1, pLevel->inP2);
}
if( pLevel->iLeftJoin ){
int addr;
addr = sqliteVdbeAddOp(v, OP_MemLoad, pLevel->iLeftJoin, 0);
sqliteVdbeAddOp(v, OP_NotNull, 1, addr+4 + (pLevel->iCur>=0));
sqliteVdbeAddOp(v, OP_NullRow, pTabList->a[i].iCursor, 0);
if( pLevel->iCur>=0 ){
sqliteVdbeAddOp(v, OP_NullRow, pLevel->iCur, 0);
}
sqliteVdbeAddOp(v, OP_Goto, 0, pLevel->top);
}
}
sqliteVdbeResolveLabel(v, pWInfo->iBreak);
for(i=0; i<pTabList->nSrc; i++){
Table *pTab = pTabList->a[i].pTab;
assert( pTab!=0 );
if( pTab->isTransient || pTab->pSelect ) continue;
pLevel = &pWInfo->a[i];
sqliteVdbeAddOp(v, OP_Close, pTabList->a[i].iCursor, 0);
if( pLevel->pIdx!=0 ){
sqliteVdbeAddOp(v, OP_Close, pLevel->iCur, 0);
}
}
#if 0 /* Never reuse a cursor */
if( pWInfo->pParse->nTab==pWInfo->peakNTab ){
pWInfo->pParse->nTab = pWInfo->savedNTab;
}
#endif
sqliteFree(pWInfo);
return;
}
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