Add a new hint bit on the flags parameter of sqlite3BtreeDelete(). The new

BTREE_IDXDELETE bit indicates that the call is to delete an index entry 
corresponding to a table row that has already been deleted.

FossilOrigin-Name: ac2cbadd8000947c097da5b00c00090fe58fdcff

5 files changed, 59 insertions, 22 deletions

diff --git a/src/btree.c b/src/btree.c
index 4e6f6478a..89144e7a8 100644
--- a/src/btree.c
+++ b/src/btree.c
@@ -4049,13 +4049,13 @@ int sqlite3BtreeSavepoint(Btree *p, int op, int iSavepoint){
 ** on the database already. If a write-cursor is requested, then
 ** the caller is assumed to have an open write transaction.
 **
-** If wrFlag==0, then the cursor can only be used for reading.
-** If wrFlag==1, then the cursor can be used for reading or for
-** writing if other conditions for writing are also met.  These
-** are the conditions that must be met in order for writing to
-** be allowed:
+** If the BTREE_WRCSR bit of wrFlag is clear, then the cursor can only
+** be used for reading.  If the BTREE_WRCSR bit is set, then the cursor
+** can be used for reading or for writing if other conditions for writing
+** are also met.  These are the conditions that must be met in order
+** for writing to be allowed:
 **
-** 1:  The cursor must have been opened with wrFlag==1
+** 1:  The cursor must have been opened with wrFlag containing BTREE_WRCSR
 **
 ** 2:  Other database connections that share the same pager cache
 **     but which are not in the READ_UNCOMMITTED state may not have
@@ -4067,6 +4067,16 @@ int sqlite3BtreeSavepoint(Btree *p, int op, int iSavepoint){
 **
 ** 4:  There must be an active transaction.
 **
+** The BTREE_FORDELETE bit of wrFlag may optionally be set if BTREE_WRCSR
+** is set.  If FORDELETE is set, that is a hint to the implementation that
+** this cursor will only be used to seek to and delete entries of an index
+** as part of a larger DELETE statement.  The FORDELETE hint is not used by
+** this implementation.  But in a hypothetical alternative storage engine 
+** in which index entries are automatically deleted when corresponding table
+** rows are deleted, the FORDELETE flag is a hint that all SEEK and DELETE
+** operations on this cursor can be no-ops and all READ operations can 
+** return a null row (2-bytes: 0x01 0x00).
+**
 ** No checking is done to make sure that page iTable really is the
 ** root page of a b-tree.  If it is not, then the cursor acquired
 ** will not work correctly.
@@ -8082,13 +8092,18 @@ end_insert:
 /*
 ** Delete the entry that the cursor is pointing to. 
 **
-** If the second parameter is zero, then the cursor is left pointing at an
-** arbitrary location after the delete. If it is non-zero, then the cursor 
-** is left in a state such that the next call to BtreeNext() or BtreePrev()
-** moves it to the same row as it would if the call to BtreeDelete() had
-** been omitted.
+** If the BTREE_SAVEPOSITION bit of the flags parameter is zero, then
+** the cursor is left pointing at an arbitrary location after the delete.
+** But if that bit is set, then the cursor is left in a state such that
+** the next call to BtreeNext() or BtreePrev() moves it to the same row
+** as it would have been on if the call to BtreeDelete() had been omitted.
+**
+** The BTREE_IDXDELETE bit of flags indicates that this is a delete of
+** an index entry where the corresponding table row has already been deleted.
+** The BTREE_IDXDELETE bit is a hint that is not used by this implementation,
+** but which might be used by alternative storage engines.
 */
-int sqlite3BtreeDelete(BtCursor *pCur, int bPreserve){
+int sqlite3BtreeDelete(BtCursor *pCur, u8 flags){
   Btree *p = pCur->pBtree;
   BtShared *pBt = p->pBt;              
   int rc;                              /* Return code */
@@ -8098,6 +8113,7 @@ int sqlite3BtreeDelete(BtCursor *pCur, int bPreserve){
   int iCellDepth;                      /* Depth of node containing pCell */ 
   u16 szCell;                          /* Size of the cell being deleted */
   int bSkipnext = 0;                   /* Leaf cursor in SKIPNEXT state */
+  u8 bPreserve = flags & BTREE_SAVEPOSITION;  /* Keep cursor valid */
 
   assert( cursorOwnsBtShared(pCur) );
   assert( pBt->inTransaction==TRANS_WRITE );
@@ -8107,6 +8123,7 @@ int sqlite3BtreeDelete(BtCursor *pCur, int bPreserve){
   assert( !hasReadConflicts(p, pCur->pgnoRoot) );
   assert( pCur->aiIdx[pCur->iPage]<pCur->apPage[pCur->iPage]->nCell );
   assert( pCur->eState==CURSOR_VALID );
+  assert( (flags & ~(BTREE_SAVEPOSITION | BTREE_IDXDELETE))==0 );
 
   iCellDepth = pCur->iPage;
   iCellIdx = pCur->aiIdx[iCellDepth];
diff --git a/src/btree.h b/src/btree.h
index 37a9915ed..416faca65 100644
--- a/src/btree.h
+++ b/src/btree.h
@@ -245,7 +245,12 @@ int sqlite3BtreeMovetoUnpacked(
 );
 int sqlite3BtreeCursorHasMoved(BtCursor*);
 int sqlite3BtreeCursorRestore(BtCursor*, int*);
-int sqlite3BtreeDelete(BtCursor*, int);
+int sqlite3BtreeDelete(BtCursor*, u8 flags);
+
+/* Allowed flags for the 2nd argument to sqlite3BtreeDelete() */
+#define BTREE_SAVEPOSITION 0x02  /* Leave cursor pointing at NEXT or PREV */
+#define BTREE_IDXDELETE    0x04  /* this is index, table row already deleted */
+
 int sqlite3BtreeInsert(BtCursor*, const void *pKey, i64 nKey,
                                   const void *pData, int nData,
                                   int nZero, int bias, int seekResult);
diff --git a/src/delete.c b/src/delete.c
index ed273bde8..98c49c993 100644
--- a/src/delete.c
+++ b/src/delete.c
@@ -718,6 +718,7 @@ void sqlite3GenerateRowDelete(
   ** a view (in which case the only effect of the DELETE statement is to
   ** fire the INSTEAD OF triggers).  */ 
   if( pTab->pSelect==0 ){
+    u8 p5 = 0;
     sqlite3GenerateRowIndexDelete(pParse, pTab, iDataCur, iIdxCur,0,iIdxNoSeek);
     sqlite3VdbeAddOp2(v, OP_Delete, iDataCur, (count?OPFLAG_NCHANGE:0));
     if( count ){
@@ -725,8 +726,10 @@ void sqlite3GenerateRowDelete(
     }
     if( iIdxNoSeek>=0 ){
       sqlite3VdbeAddOp1(v, OP_Delete, iIdxNoSeek);
+      if( eMode!=ONEPASS_OFF ) p5 = OPFLAG_IDXDELETE;
     }
-    sqlite3VdbeChangeP5(v, eMode==ONEPASS_MULTI);
+    if( eMode==ONEPASS_MULTI ) p5 |= OPFLAG_SAVEPOSITION;
+    sqlite3VdbeChangeP5(v, p5);
   }
 
   /* Do any ON CASCADE, SET NULL or SET DEFAULT operations required to
diff --git a/src/sqliteInt.h b/src/sqliteInt.h
index b536b8ab2..25aff0015 100644
--- a/src/sqliteInt.h
+++ b/src/sqliteInt.h
@@ -2847,7 +2847,8 @@ struct AuthContext {
 /*
 ** Bitfield flags for P5 value in various opcodes.
 */
-#define OPFLAG_NCHANGE       0x01    /* Set to update db->nChange */
+#define OPFLAG_NCHANGE       0x01    /* OP_Insert: Set to update db->nChange */
+                                     /* Also used in P2 (not P5) of OP_Delete */
 #define OPFLAG_EPHEM         0x01    /* OP_Column: Ephemeral output is ok */
 #define OPFLAG_LASTROWID     0x02    /* Set to update db->lastRowid */
 #define OPFLAG_ISUPDATE      0x04    /* This OP_Insert is an sql UPDATE */
@@ -2860,6 +2861,8 @@ struct AuthContext {
 #define OPFLAG_FORDELETE     0x08    /* OP_Open should use BTREE_FORDELETE */
 #define OPFLAG_P2ISREG       0x10    /* P2 to OP_Open** is a register number */
 #define OPFLAG_PERMUTE       0x01    /* OP_Compare: use the permutation */
+#define OPFLAG_SAVEPOSITION  0x02    /* OP_Delete: keep cursor position */
+#define OPFLAG_IDXDELETE     0x04    /* OP_Delete: index in a DELETE op */
 
 /*
  * Each trigger present in the database schema is stored as an instance of
diff --git a/src/vdbe.c b/src/vdbe.c
index c6d5f7b0c..c290097da 100644
--- a/src/vdbe.c
+++ b/src/vdbe.c
@@ -4341,14 +4341,20 @@ case OP_InsertInt: {
 **
 ** Delete the record at which the P1 cursor is currently pointing.
 **
-** If the P5 parameter is non-zero, the cursor will be left pointing at 
-** either the next or the previous record in the table. If it is left 
-** pointing at the next record, then the next Next instruction will be a 
-** no-op. As a result, in this case it is OK to delete a record from within a
-** Next loop. If P5 is zero, then the cursor is left in an undefined state.
+** If the OPFLAG_SAVEPOSITION bit of the P5 parameter is set, then
+** the cursor will be left pointing at  either the next or the previous
+** record in the table. If it is left pointing at the next record, then
+** the next Next instruction will be a no-op. As a result, in this case
+** it is ok to delete a record from within a Next loop. If 
+** OPFLAG_SAVEPOSITION bit of P5 is clear, then the cursor will be
+** left in an undefined state.
 **
-** If the OPFLAG_NCHANGE flag of P2 is set, then the row change count is
-** incremented (otherwise not).
+** If the OPFLAG_IDXDELETE bit is set on P5, that indicates that this
+** delete is on an index cursor where the corresponding table row has
+** already been deleted.
+**
+** If the OPFLAG_NCHANGE flag of P2 (NB: P2 not P5) is set, then the row
+** change count is incremented (otherwise not).
 **
 ** P1 must not be pseudo-table.  It has to be a real table with
 ** multiple rows.
@@ -4385,6 +4391,9 @@ case OP_Delete: {
   }
 #endif
  
+  assert( (pOp->p5 & ~(OPFLAG_SAVEPOSITION|OPFLAG_IDXDELETE))==0 );
+  assert( OPFLAG_SAVEPOSITION==BTREE_SAVEPOSITION );
+  assert( OPFLAG_IDXDELETE==BTREE_IDXDELETE );
   rc = sqlite3BtreeDelete(pC->uc.pCursor, pOp->p5);
   pC->cacheStatus = CACHE_STALE;