Proposed optimization to the IS NULL and NOT NULL operators that avoids

loading the entire content of larges strings and BLOBs.  Response to
[forum:/info/3c08d4715dc05b00|forum post 3c08d4715dc05b00].

FossilOrigin-Name: 45f171565442f9fd6574fb93ae7abe83c168b20be68af42531bc55571450d3ab

4 files changed, 20 insertions, 3 deletions

diff --git a/src/expr.c b/src/expr.c
index baa0fe647..bceb5efb0 100644
--- a/src/expr.c
+++ b/src/expr.c
@@ -5241,6 +5241,7 @@ void sqlite3ExprIfTrue(Parse *pParse, Expr *pExpr, int dest, int jumpIfNull){
       assert( TK_ISNULL==OP_IsNull );   testcase( op==TK_ISNULL );
       assert( TK_NOTNULL==OP_NotNull ); testcase( op==TK_NOTNULL );
       r1 = sqlite3ExprCodeTemp(pParse, pExpr->pLeft, &regFree1);
+      sqlite3VdbeTypeofColumn(v, r1);
       sqlite3VdbeAddOp2(v, op, r1, dest);
       VdbeCoverageIf(v, op==TK_ISNULL);
       VdbeCoverageIf(v, op==TK_NOTNULL);
@@ -5415,6 +5416,7 @@ void sqlite3ExprIfFalse(Parse *pParse, Expr *pExpr, int dest, int jumpIfNull){
     case TK_ISNULL:
     case TK_NOTNULL: {
       r1 = sqlite3ExprCodeTemp(pParse, pExpr->pLeft, &regFree1);
+      sqlite3VdbeTypeofColumn(v, r1);
       sqlite3VdbeAddOp2(v, op, r1, dest);
       testcase( op==TK_ISNULL );   VdbeCoverageIf(v, op==TK_ISNULL);
       testcase( op==TK_NOTNULL );  VdbeCoverageIf(v, op==TK_NOTNULL);
diff --git a/src/pragma.c b/src/pragma.c
index 7976f83c6..7aea3dd2c 100644
--- a/src/pragma.c
+++ b/src/pragma.c
@@ -1791,9 +1791,7 @@ void sqlite3Pragma(
                 || pTab->iPKey==mxCol) ) mxCol--;
         if( mxCol>=0 ){
           sqlite3ExprCodeGetColumnOfTable(v, pTab, iDataCur, mxCol, 3);
-          if( sqlite3VdbeGetLastOp(v)->opcode==OP_Column ){
-            sqlite3VdbeChangeP5(v, OPFLAG_TYPEOFARG);
-          }
+          sqlite3VdbeTypeofColumn(v, 3);
         }
 
         if( !isQuick ){
diff --git a/src/vdbe.h b/src/vdbe.h
index eb1445f1d..a244468b5 100644
--- a/src/vdbe.h
+++ b/src/vdbe.h
@@ -228,6 +228,7 @@ void sqlite3VdbeChangeP1(Vdbe*, int addr, int P1);
 void sqlite3VdbeChangeP2(Vdbe*, int addr, int P2);
 void sqlite3VdbeChangeP3(Vdbe*, int addr, int P3);
 void sqlite3VdbeChangeP5(Vdbe*, u16 P5);
+void sqlite3VdbeTypeofColumn(Vdbe*, int);
 void sqlite3VdbeJumpHere(Vdbe*, int addr);
 void sqlite3VdbeJumpHereOrPopInst(Vdbe*, int addr);
 int sqlite3VdbeChangeToNoop(Vdbe*, int addr);
diff --git a/src/vdbeaux.c b/src/vdbeaux.c
index 8785e3b56..275ba6f77 100644
--- a/src/vdbeaux.c
+++ b/src/vdbeaux.c
@@ -1156,6 +1156,22 @@ void sqlite3VdbeChangeP5(Vdbe *p, u16 p5){
 }
 
 /*
+** If the previous opcode is an OP_Column that delivers results
+** into register iDest, then add the OPFLAG_TYPEOF flag to that
+** opcode.
+*/
+void sqlite3VdbeTypeofColumn(Vdbe *p, int iDest){
+  if( p->nOp>0 ){
+    VdbeOp *pOp = &p->aOp[p->nOp-1];
+    if( pOp->opcode==OP_Column && pOp->p3==iDest ){
+      pOp->p5 |= OPFLAG_TYPEOFARG;
+    }
+  }else{
+    assert( p->db->mallocFailed );
+  }
+}
+
+/*
 ** Change the P2 operand of instruction addr so that it points to
 ** the address of the next instruction to be coded.
 */