Enhance the code generator for INSERT INTO ... SELECT so that the SELECT

generates output directly in the registers that INSERT INTO will be using,
in many cases, and OP_SCopy operations can thus be avoided.

FossilOrigin-Name: aa2d8b0e8154dd2f5e2c837dc11ab362b083495b

5 files changed, 148 insertions, 185 deletions

diff --git a/src/expr.c b/src/expr.c
index 0f0718101..79b50fbb3 100644
--- a/src/expr.c
+++ b/src/expr.c
@@ -3112,7 +3112,7 @@ int sqlite3ExprCodeTemp(Parse *pParse, Expr *pExpr, int *pReg){
 ** results in register target.  The results are guaranteed to appear
 ** in register target.
 */
-int sqlite3ExprCode(Parse *pParse, Expr *pExpr, int target){
+void sqlite3ExprCode(Parse *pParse, Expr *pExpr, int target){
   int inReg;
 
   assert( target>0 && target<=pParse->nMem );
@@ -3125,7 +3125,20 @@ int sqlite3ExprCode(Parse *pParse, Expr *pExpr, int target){
       sqlite3VdbeAddOp2(pParse->pVdbe, OP_SCopy, inReg, target);
     }
   }
-  return target;
+}
+
+/*
+** Generate code that will evaluate expression pExpr and store the
+** results in register target.  The results are guaranteed to appear
+** in register target.  If the expression is constant, then this routine
+** might choose to code the expression at initialization time.
+*/
+void sqlite3ExprCodeFactorable(Parse *pParse, Expr *pExpr, int target){
+  if( pParse->okConstFactor && sqlite3ExprIsConstant(pExpr) ){
+    sqlite3ExprCodeAtInit(pParse, pExpr, target, 0);
+  }else{
+    sqlite3ExprCode(pParse, pExpr, target);
+  }
 }
 
 /*
@@ -3140,25 +3153,16 @@ int sqlite3ExprCode(Parse *pParse, Expr *pExpr, int target){
 ** times.  They are evaluated once and the results of the expression
 ** are reused.
 */
-int sqlite3ExprCodeAndCache(Parse *pParse, Expr *pExpr, int target){
+void sqlite3ExprCodeAndCache(Parse *pParse, Expr *pExpr, int target){
   Vdbe *v = pParse->pVdbe;
-  int inReg;
-  inReg = sqlite3ExprCode(pParse, pExpr, target);
+  int iMem;
+
   assert( target>0 );
-  /* The only place, other than this routine, where expressions can be
-  ** converted to TK_REGISTER is internal subexpressions in BETWEEN and
-  ** CASE operators.  Neither ever calls this routine.  And this routine
-  ** is never called twice on the same expression.  Hence it is impossible
-  ** for the input to this routine to already be a register.  Nevertheless,
-  ** it seems prudent to keep the ALWAYS() in case the conditions above
-  ** change with future modifications or enhancements. */
-  if( ALWAYS(pExpr->op!=TK_REGISTER) ){  
-    int iMem;
-    iMem = ++pParse->nMem;
-    sqlite3VdbeAddOp2(v, OP_Copy, inReg, iMem);
-    exprToRegister(pExpr, iMem);
-  }
-  return inReg;
+  assert( pExpr->op!=TK_REGISTER );
+  sqlite3ExprCode(pParse, pExpr, target);
+  iMem = ++pParse->nMem;
+  sqlite3VdbeAddOp2(v, OP_Copy, target, iMem);
+  exprToRegister(pExpr, iMem);
 }
 
 #if defined(SQLITE_ENABLE_TREE_EXPLAIN)
diff --git a/src/insert.c b/src/insert.c
index bd15832ac..a9e6bd286 100644
--- a/src/insert.c
+++ b/src/insert.c
@@ -148,7 +148,7 @@ void sqlite3TableAffinityStr(Vdbe *v, Table *pTab){
 ** a statement of the form  "INSERT INTO <iDb, pTab> SELECT ..." can 
 ** run without using temporary table for the results of the SELECT. 
 */
-static int readsTable(Parse *p, int iStartAddr, int iDb, Table *pTab){
+static int readsTable(Parse *p, int iDb, Table *pTab){
   Vdbe *v = sqlite3GetVdbe(p);
   int i;
   int iEnd = sqlite3VdbeCurrentAddr(v);
@@ -156,7 +156,7 @@ static int readsTable(Parse *p, int iStartAddr, int iDb, Table *pTab){
   VTable *pVTab = IsVirtual(pTab) ? sqlite3GetVTable(p->db, pTab) : 0;
 #endif
 
-  for(i=iStartAddr; i<iEnd; i++){
+  for(i=1; i<iEnd; i++){
     VdbeOp *pOp = sqlite3VdbeGetOp(v, i);
     assert( pOp!=0 );
     if( pOp->opcode==OP_OpenRead && pOp->p3==iDb ){
@@ -335,79 +335,6 @@ void sqlite3AutoincrementEnd(Parse *pParse){
 #endif /* SQLITE_OMIT_AUTOINCREMENT */
 
 
-/*
-** Generate code for a co-routine that will evaluate a subquery one
-** row at a time.
-**
-** The pSelect parameter is the subquery that the co-routine will evaluation.
-** Information about the location of co-routine and the registers it will use
-** is returned by filling in the pDest object.
-**
-** Registers are allocated as follows:
-**
-**   pDest->iSDParm      The register holding the next entry-point of the
-**                       co-routine.  Run the co-routine to its next breakpoint
-**                       by calling "OP_Yield $X" where $X is pDest->iSDParm.
-**
-**   pDest->iSdst        First result register.
-**
-**   pDest->nSdst        Number of result registers.
-**
-** At EOF the first result register will be marked as "undefined" so that
-** the caller can know when to stop reading results.
-**
-** This routine handles all of the register allocation and fills in the
-** pDest structure appropriately.
-**
-** Here is a schematic of the generated code assuming that X is the 
-** co-routine entry-point register reg[pDest->iSDParm], that EOF is the
-** completed flag reg[pDest->iSDParm+1], and R and S are the range of
-** registers that hold the result set, reg[pDest->iSdst] through
-** reg[pDest->iSdst+pDest->nSdst-1]:
-**
-**         X <- A
-**         goto B
-**      A: setup for the SELECT
-**         loop rows in the SELECT
-**           load results into registers R..S
-**           yield X
-**         end loop
-**         cleanup after the SELECT
-**         end co-routine R
-**      B:
-**
-** To use this subroutine, the caller generates code as follows:
-**
-**         [ Co-routine generated by this subroutine, shown above ]
-**      S: yield X, at EOF goto E
-**         if skip this row, goto C
-**         if terminate loop, goto E
-**         deal with this row
-**      C: goto S
-**      E:
-*/
-int sqlite3CodeCoroutine(Parse *pParse, Select *pSelect, SelectDest *pDest){
-  int regYield;       /* Register holding co-routine entry-point */
-  int addrTop;        /* Top of the co-routine */
-  int rc;             /* Result code */
-  Vdbe *v;            /* VDBE under construction */
-
-  regYield = ++pParse->nMem;
-  v = sqlite3GetVdbe(pParse);
-  addrTop = sqlite3VdbeCurrentAddr(v) + 1;
-  sqlite3VdbeAddOp3(v, OP_InitCoroutine, regYield, 0, addrTop);
-  sqlite3SelectDestInit(pDest, SRT_Coroutine, regYield);
-  rc = sqlite3Select(pParse, pSelect, pDest);
-  assert( pParse->nErr==0 || rc );
-  if( pParse->db->mallocFailed && rc==SQLITE_OK ) rc = SQLITE_NOMEM;
-  if( rc ) return rc;
-  sqlite3VdbeAddOp1(v, OP_EndCoroutine, regYield);
-  sqlite3VdbeJumpHere(v, addrTop - 1);                       /* label B: */
-  return rc;
-}
-
-
-
 /* Forward declaration */
 static int xferOptimization(
   Parse *pParse,        /* Parser context */
@@ -531,16 +458,16 @@ void sqlite3Insert(
   int iIdxCur = 0;      /* First index cursor */
   int ipkColumn = -1;   /* Column that is the INTEGER PRIMARY KEY */
   int endOfLoop;        /* Label for the end of the insertion loop */
-  int useTempTable = 0; /* Store SELECT results in intermediate table */
   int srcTab = 0;       /* Data comes from this temporary cursor if >=0 */
   int addrInsTop = 0;   /* Jump to label "D" */
   int addrCont = 0;     /* Top of insert loop. Label "C" in templates 3 and 4 */
-  int addrSelect = 0;   /* Address of coroutine that implements the SELECT */
   SelectDest dest;      /* Destination for SELECT on rhs of INSERT */
   int iDb;              /* Index of database holding TABLE */
   Db *pDb;              /* The database containing table being inserted into */
-  int appendFlag = 0;   /* True if the insert is likely to be an append */
-  int withoutRowid;     /* 0 for normal table.  1 for WITHOUT ROWID table */
+  u8 useTempTable = 0;  /* Store SELECT results in intermediate table */
+  u8 appendFlag = 0;    /* True if the insert is likely to be an append */
+  u8 withoutRowid;      /* 0 for normal table.  1 for WITHOUT ROWID table */
+  u8 bIdListInOrder = 1; /* True if IDLIST is in table order */
   ExprList *pList = 0;  /* List of VALUES() to be inserted  */
 
   /* Register allocations */
@@ -652,6 +579,56 @@ void sqlite3Insert(
   */
   regAutoinc = autoIncBegin(pParse, iDb, pTab);
 
+  /* Allocate registers for holding the rowid of the new row,
+  ** the content of the new row, and the assemblied row record.
+  */
+  regRowid = regIns = pParse->nMem+1;
+  pParse->nMem += pTab->nCol + 1;
+  if( IsVirtual(pTab) ){
+    regRowid++;
+    pParse->nMem++;
+  }
+  regData = regRowid+1;
+
+  /* If the INSERT statement included an IDLIST term, then make sure
+  ** all elements of the IDLIST really are columns of the table and 
+  ** remember the column indices.
+  **
+  ** If the table has an INTEGER PRIMARY KEY column and that column
+  ** is named in the IDLIST, then record in the ipkColumn variable
+  ** the index into IDLIST of the primary key column.  ipkColumn is
+  ** the index of the primary key as it appears in IDLIST, not as
+  ** is appears in the original table.  (The index of the INTEGER
+  ** PRIMARY KEY in the original table is pTab->iPKey.)
+  */
+  if( pColumn ){
+    for(i=0; i<pColumn->nId; i++){
+      pColumn->a[i].idx = -1;
+    }
+    for(i=0; i<pColumn->nId; i++){
+      for(j=0; j<pTab->nCol; j++){
+        if( sqlite3StrICmp(pColumn->a[i].zName, pTab->aCol[j].zName)==0 ){
+          pColumn->a[i].idx = j;
+          if( i!=j ) bIdListInOrder = 0;
+          if( j==pTab->iPKey ){
+            ipkColumn = i;  assert( !withoutRowid );
+          }
+          break;
+        }
+      }
+      if( j>=pTab->nCol ){
+        if( sqlite3IsRowid(pColumn->a[i].zName) && !withoutRowid ){
+          ipkColumn = i;
+        }else{
+          sqlite3ErrorMsg(pParse, "table %S has no column named %s",
+              pTabList, 0, pColumn->a[i].zName);
+          pParse->checkSchema = 1;
+          goto insert_cleanup;
+        }
+      }
+    }
+  }
+
   /* Figure out how many columns of data are supplied.  If the data
   ** is coming from a SELECT statement, then generate a co-routine that
   ** produces a single row of the SELECT on each invocation.  The
@@ -659,13 +636,24 @@ void sqlite3Insert(
   */
   if( pSelect ){
     /* Data is coming from a SELECT.  Generate a co-routine to run the SELECT */
-    int rc = sqlite3CodeCoroutine(pParse, pSelect, &dest);
-    if( rc ) goto insert_cleanup;
-
+    int regYield;       /* Register holding co-routine entry-point */
+    int addrTop;        /* Top of the co-routine */
+    int rc;             /* Result code */
+
+    regYield = ++pParse->nMem;
+    addrTop = sqlite3VdbeCurrentAddr(v) + 1;
+    sqlite3VdbeAddOp3(v, OP_InitCoroutine, regYield, 0, addrTop);
+    sqlite3SelectDestInit(&dest, SRT_Coroutine, regYield);
+    dest.iSdst = bIdListInOrder ? regData : 0;
+    dest.nSdst = pTab->nCol;
+    rc = sqlite3Select(pParse, pSelect, &dest);
     regFromSelect = dest.iSdst;
+    assert( pParse->nErr==0 || rc );
+    if( rc || db->mallocFailed ) goto insert_cleanup;
+    sqlite3VdbeAddOp1(v, OP_EndCoroutine, regYield);
+    sqlite3VdbeJumpHere(v, addrTop - 1);                       /* label B: */
     assert( pSelect->pEList );
     nColumn = pSelect->pEList->nExpr;
-    assert( dest.nSdst==nColumn );
 
     /* Set useTempTable to TRUE if the result of the SELECT statement
     ** should be written into a temporary table (template 4).  Set to
@@ -676,7 +664,7 @@ void sqlite3Insert(
     ** of the tables being read by the SELECT statement.  Also use a 
     ** temp table in the case of row triggers.
     */
-    if( pTrigger || readsTable(pParse, addrSelect, iDb, pTab) ){
+    if( pTrigger || readsTable(pParse, iDb, pTab) ){
       useTempTable = 1;
     }
 
@@ -725,6 +713,14 @@ void sqlite3Insert(
     }
   }
 
+  /* If there is no IDLIST term but the table has an integer primary
+  ** key, the set the ipkColumn variable to the integer primary key 
+  ** column index in the original table definition.
+  */
+  if( pColumn==0 && nColumn>0 ){
+    ipkColumn = pTab->iPKey;
+  }
+
   /* Make sure the number of columns in the source data matches the number
   ** of columns to be inserted into the table.
   */
@@ -743,52 +739,6 @@ void sqlite3Insert(
     sqlite3ErrorMsg(pParse, "%d values for %d columns", nColumn, pColumn->nId);
     goto insert_cleanup;
   }
-
-  /* If the INSERT statement included an IDLIST term, then make sure
-  ** all elements of the IDLIST really are columns of the table and 
-  ** remember the column indices.
-  **
-  ** If the table has an INTEGER PRIMARY KEY column and that column
-  ** is named in the IDLIST, then record in the ipkColumn variable
-  ** the index into IDLIST of the primary key column.  ipkColumn is
-  ** the index of the primary key as it appears in IDLIST, not as
-  ** is appears in the original table.  (The index of the INTEGER
-  ** PRIMARY KEY in the original table is pTab->iPKey.)
-  */
-  if( pColumn ){
-    for(i=0; i<pColumn->nId; i++){
-      pColumn->a[i].idx = -1;
-    }
-    for(i=0; i<pColumn->nId; i++){
-      for(j=0; j<pTab->nCol; j++){
-        if( sqlite3StrICmp(pColumn->a[i].zName, pTab->aCol[j].zName)==0 ){
-          pColumn->a[i].idx = j;
-          if( j==pTab->iPKey ){
-            ipkColumn = i;  assert( !withoutRowid );
-          }
-          break;
-        }
-      }
-      if( j>=pTab->nCol ){
-        if( sqlite3IsRowid(pColumn->a[i].zName) && !withoutRowid ){
-          ipkColumn = i;
-        }else{
-          sqlite3ErrorMsg(pParse, "table %S has no column named %s",
-              pTabList, 0, pColumn->a[i].zName);
-          pParse->checkSchema = 1;
-          goto insert_cleanup;
-        }
-      }
-    }
-  }
-
-  /* If there is no IDLIST term but the table has an integer primary
-  ** key, the set the ipkColumn variable to the integer primary key 
-  ** column index in the original table definition.
-  */
-  if( pColumn==0 && nColumn>0 ){
-    ipkColumn = pTab->iPKey;
-  }
     
   /* Initialize the count of rows to be inserted
   */
@@ -836,17 +786,6 @@ void sqlite3Insert(
     addrInsTop = addrCont = sqlite3VdbeAddOp1(v, OP_Yield, dest.iSDParm);
   }
 
-  /* Allocate registers for holding the rowid of the new row,
-  ** the content of the new row, and the assemblied row record.
-  */
-  regRowid = regIns = pParse->nMem+1;
-  pParse->nMem += pTab->nCol + 1;
-  if( IsVirtual(pTab) ){
-    regRowid++;
-    pParse->nMem++;
-  }
-  regData = regRowid+1;
-
   /* Run the BEFORE and INSTEAD OF triggers, if there are any
   */
   endOfLoop = sqlite3VdbeMakeLabel(v);
@@ -930,7 +869,7 @@ void sqlite3Insert(
       if( useTempTable ){
         sqlite3VdbeAddOp3(v, OP_Column, srcTab, ipkColumn, regRowid);
       }else if( pSelect ){
-        sqlite3VdbeAddOp2(v, OP_SCopy, regFromSelect+ipkColumn, regRowid);
+        sqlite3VdbeAddOp2(v, OP_Copy, regFromSelect+ipkColumn, regRowid);
       }else{
         VdbeOp *pOp;
         sqlite3ExprCode(pParse, pList->a[ipkColumn].pExpr, regRowid);
@@ -976,8 +915,9 @@ void sqlite3Insert(
         /* The value of the INTEGER PRIMARY KEY column is always a NULL.
         ** Whenever this column is read, the rowid will be substituted
         ** in its place.  Hence, fill this column with a NULL to avoid
-        ** taking up data space with information that will never be used. */
-        sqlite3VdbeAddOp2(v, OP_Null, 0, iRegStore);
+        ** taking up data space with information that will never be used.
+        ** As there may be shallow copies of this value, make it a soft-NULL */
+        sqlite3VdbeAddOp1(v, OP_SoftNull, iRegStore);
         continue;
       }
       if( pColumn==0 ){
@@ -994,11 +934,13 @@ void sqlite3Insert(
         }
       }
       if( j<0 || nColumn==0 || (pColumn && j>=pColumn->nId) ){
-        sqlite3ExprCode(pParse, pTab->aCol[i].pDflt, iRegStore);
+        sqlite3ExprCodeFactorable(pParse, pTab->aCol[i].pDflt, iRegStore);
       }else if( useTempTable ){
         sqlite3VdbeAddOp3(v, OP_Column, srcTab, j, iRegStore); 
       }else if( pSelect ){
-        sqlite3VdbeAddOp2(v, OP_SCopy, regFromSelect+j, iRegStore);
+        if( regFromSelect!=regData ){
+          sqlite3VdbeAddOp2(v, OP_SCopy, regFromSelect+j, iRegStore);
+        }
       }else{
         sqlite3ExprCode(pParse, pList->a[j].pExpr, iRegStore);
       }
diff --git a/src/select.c b/src/select.c
index dd445deed..a41f4a825 100644
--- a/src/select.c
+++ b/src/select.c
@@ -585,13 +585,19 @@ static void selectInnerLoop(
   /* Pull the requested columns.
   */
   nResultCol = pEList->nExpr;
+
   if( pDest->iSdst==0 ){
     pDest->iSdst = pParse->nMem+1;
-    pDest->nSdst = nResultCol;
     pParse->nMem += nResultCol;
-  }else{ 
-    assert( pDest->nSdst==nResultCol );
+  }else if( pDest->iSdst+nResultCol > pParse->nMem ){
+    /* This is an error condition that can result, for example, when a SELECT
+    ** on the right-hand side of an INSERT contains more result columns than
+    ** there are columns in the table on the left.  The error will be caught
+    ** and reported later.  But we need to make sure enough memory is allocated
+    ** to avoid other spurious errors in the meantime. */
+    pParse->nMem += nResultCol;
   }
+  pDest->nSdst = nResultCol;
   regResult = pDest->iSdst;
   if( srcTab>=0 ){
     for(i=0; i<nResultCol; i++){
diff --git a/src/sqliteInt.h b/src/sqliteInt.h
index 8b9337a2b..a82b2e737 100644
--- a/src/sqliteInt.h
+++ b/src/sqliteInt.h
@@ -2990,7 +2990,6 @@ void sqlite3DeleteTable(sqlite3*, Table*);
 # define sqlite3AutoincrementBegin(X)
 # define sqlite3AutoincrementEnd(X)
 #endif
-int sqlite3CodeCoroutine(Parse*, Select*, SelectDest*);
 void sqlite3Insert(Parse*, SrcList*, Select*, IdList*, int);
 void *sqlite3ArrayAllocate(sqlite3*,void*,int,int*,int*);
 IdList *sqlite3IdListAppend(sqlite3*, IdList*, Token*);
@@ -3038,11 +3037,12 @@ void sqlite3ExprCachePop(Parse*, int);
 void sqlite3ExprCacheRemove(Parse*, int, int);
 void sqlite3ExprCacheClear(Parse*);
 void sqlite3ExprCacheAffinityChange(Parse*, int, int);
-int sqlite3ExprCode(Parse*, Expr*, int);
+void sqlite3ExprCode(Parse*, Expr*, int);
+void sqlite3ExprCodeFactorable(Parse*, Expr*, int);
 void sqlite3ExprCodeAtInit(Parse*, Expr*, int, u8);
 int sqlite3ExprCodeTemp(Parse*, Expr*, int*);
 int sqlite3ExprCodeTarget(Parse*, Expr*, int);
-int sqlite3ExprCodeAndCache(Parse*, Expr*, int);
+void sqlite3ExprCodeAndCache(Parse*, Expr*, int);
 int sqlite3ExprCodeExprList(Parse*, ExprList*, int, u8);
 #define SQLITE_ECEL_DUP      0x01  /* Deep, not shallow copies */
 #define SQLITE_ECEL_FACTOR   0x02  /* Factor out constant terms */
diff --git a/src/vdbe.c b/src/vdbe.c
index 610376dd2..41ad66215 100644
--- a/src/vdbe.c
+++ b/src/vdbe.c
@@ -994,6 +994,20 @@ case OP_Null: {           /* out2-prerelease */
   break;
 }
 
+/* Opcode: SoftNull P1 * * * *
+** Synopsis:  r[P1]=NULL
+**
+** Set register P1 to have the value NULL as seen by the OP_MakeRecord
+** instruction, but do not free any string or blob memory associated with
+** the register, so that if the value was a string or blob that was
+** previously copied using OP_SCopy, the copies will continue to be valid.
+*/
+case OP_SoftNull: {
+  assert( pOp->p1>0 && pOp->p1<=(p->nMem-p->nCursor) );
+  pOut = &aMem[pOp->p1];
+  pOut->flags = (pOut->flags|MEM_Null)&~MEM_Undefined;
+  break;
+}
 
 /* Opcode: Blob P1 P2 * P4 *
 ** Synopsis: r[P2]=P4 (len=P1)
@@ -2909,23 +2923,17 @@ case OP_AutoCommit: {
 
 /* Opcode: Transaction P1 P2 P3 P4 P5
 **
-** Begin a transaction.  The transaction ends when a Commit or Rollback
-** opcode is encountered.  Depending on the ON CONFLICT setting, the
-** transaction might also be rolled back if an error is encountered.
+** Begin a transaction on database P1 if a transaction is not already
+** active.
+** If P2 is non-zero, then a write-transaction is started, or if a 
+** read-transaction is already active, it is upgraded to a write-transaction.
+** If P2 is zero, then a read-transaction is started.
 **
 ** P1 is the index of the database file on which the transaction is
 ** started.  Index 0 is the main database file and index 1 is the
 ** file used for temporary tables.  Indices of 2 or more are used for
 ** attached databases.
 **
-** If P2 is non-zero, then a write-transaction is started.  A RESERVED lock is
-** obtained on the database file when a write-transaction is started.  No
-** other process can start another write transaction while this transaction is
-** underway.  Starting a write transaction also creates a rollback journal. A
-** write transaction must be started before any changes can be made to the
-** database.  If P2 is greater than or equal to 2 then an EXCLUSIVE lock is
-** also obtained on the file.
-**
 ** If a write-transaction is started and the Vdbe.usesStmtJournal flag is
 ** true (this flag is set if the Vdbe may modify more than one row and may
 ** throw an ABORT exception), a statement transaction may also be opened.
@@ -2936,13 +2944,16 @@ case OP_AutoCommit: {
 ** entire transaction. If no error is encountered, the statement transaction
 ** will automatically commit when the VDBE halts.
 **
-** If P2 is zero, then a read-lock is obtained on the database file.
-**
 ** If P5!=0 then this opcode also checks the schema cookie against P3
 ** and the schema generation counter against P4.
 ** The cookie changes its value whenever the database schema changes.
 ** This operation is used to detect when that the cookie has changed
-** and that the current process needs to reread the schema.
+** and that the current process needs to reread the schema.  If the schema
+** cookie in P3 differs from the schema cookie in the database header or
+** if the schema generation counter in P4 differs from the current
+** generation counter, then an SQLITE_SCHEMA error is raised and execution
+** halts.  The sqlite3_step() wrapper function might then reprepare the
+** statement and rerun it from the beginning.
 */
 case OP_Transaction: {
   Btree *pBt;