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SQLite training in Houston TX on 2019-11-05 (details)
Part of the 2019 Tcl Conference

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Overview
Comment:Change the OP_Last opcode so that it is a no-op if the cursor is already pointing at the end of the b-tree.
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SHA1: 663473850c4274270445b3771911fa773a8c405f
User & Date: drh 2016-11-15 04:00:24
References
2017-08-01
18:21 New ticket [cb91bf42] Assertion fault following PRAGMA reverse_unordered_selects=ON. artifact: b9784104 user: drh
Context
2016-11-15
21:17
Add a needed #include to speedtest1.c. check-in: 77aeb705 user: drh tags: trunk
17:37
Experimental changes toward making snapshots serializable. check-in: b6a81fa1 user: drh tags: serializable-snapshot
04:00
Change the OP_Last opcode so that it is a no-op if the cursor is already pointing at the end of the b-tree. check-in: 66347385 user: drh tags: trunk
02:46
Make the VACUUM command about 9% faster by avoiding unnecessary calls to sqlite3BtreeMovetoUnpacked() while copying rowid tables. check-in: 5ed0bd38 user: drh tags: trunk
Changes
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Changes to src/btree.c.

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** that is currently pointing to a row in a (non-empty) table.
** This is a verification routine is used only within assert() statements.
*/
int sqlite3BtreeCursorIsValid(BtCursor *pCur){
  return pCur && pCur->eState==CURSOR_VALID;
}
#endif /* NDEBUG */





/*
** Return the value of the integer key or "rowid" for a table btree.
** This routine is only valid for a cursor that is pointing into a
** ordinary table btree.  If the cursor points to an index btree or
** is invalid, the result of this routine is undefined.
*/







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** that is currently pointing to a row in a (non-empty) table.
** This is a verification routine is used only within assert() statements.
*/
int sqlite3BtreeCursorIsValid(BtCursor *pCur){
  return pCur && pCur->eState==CURSOR_VALID;
}
#endif /* NDEBUG */
int sqlite3BtreeCursorIsValidNN(BtCursor *pCur){
  assert( pCur!=0 );
  return pCur->eState==CURSOR_VALID;
}

/*
** Return the value of the integer key or "rowid" for a table btree.
** This routine is only valid for a cursor that is pointing into a
** ordinary table btree.  If the cursor points to an index btree or
** is invalid, the result of this routine is undefined.
*/

Changes to src/btree.h.

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int sqlite3BtreeCursorHasHint(BtCursor*, unsigned int mask);
int sqlite3BtreeIsReadonly(Btree *pBt);
int sqlite3HeaderSizeBtree(void);

#ifndef NDEBUG
int sqlite3BtreeCursorIsValid(BtCursor*);
#endif


#ifndef SQLITE_OMIT_BTREECOUNT
int sqlite3BtreeCount(BtCursor *, i64 *);
#endif

#ifdef SQLITE_TEST
int sqlite3BtreeCursorInfo(BtCursor*, int*, int);







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int sqlite3BtreeCursorHasHint(BtCursor*, unsigned int mask);
int sqlite3BtreeIsReadonly(Btree *pBt);
int sqlite3HeaderSizeBtree(void);

#ifndef NDEBUG
int sqlite3BtreeCursorIsValid(BtCursor*);
#endif
int sqlite3BtreeCursorIsValidNN(BtCursor*);

#ifndef SQLITE_OMIT_BTREECOUNT
int sqlite3BtreeCount(BtCursor *, i64 *);
#endif

#ifdef SQLITE_TEST
int sqlite3BtreeCursorInfo(BtCursor*, int*, int);

Changes to src/vdbe.c.

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** If the table or index is empty and P2>0, then jump immediately to P2.
** If P2 is 0 or if the table or index is not empty, fall through
** to the following instruction.
**
** This opcode leaves the cursor configured to move in reverse order,
** from the end toward the beginning.  In other words, the cursor is
** configured to use Prev, not Next.







*/
case OP_Last: {        /* jump */
  VdbeCursor *pC;
  BtCursor *pCrsr;
  int res;

  assert( pOp->p1>=0 && pOp->p1<p->nCursor );
  pC = p->apCsr[pOp->p1];
  assert( pC!=0 );
  assert( pC->eCurType==CURTYPE_BTREE );
  pCrsr = pC->uc.pCursor;
  res = 0;
  assert( pCrsr!=0 );
  rc = sqlite3BtreeLast(pCrsr, &res);
  pC->nullRow = (u8)res;
  pC->deferredMoveto = 0;
  pC->cacheStatus = CACHE_STALE;
  pC->seekResult = pOp->p3;
#ifdef SQLITE_DEBUG
  pC->seekOp = OP_Last;
#endif





  if( rc ) goto abort_due_to_error;
  if( pOp->p2>0 ){
    VdbeBranchTaken(res!=0,2);
    if( res ) goto jump_to_p2;



  }
  break;
}


/* Opcode: Sort P1 P2 * * *
**







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** If the table or index is empty and P2>0, then jump immediately to P2.
** If P2 is 0 or if the table or index is not empty, fall through
** to the following instruction.
**
** This opcode leaves the cursor configured to move in reverse order,
** from the end toward the beginning.  In other words, the cursor is
** configured to use Prev, not Next.
**
** If P3 is -1, then the cursor is positioned at the end of the btree
** for the purpose of appending a new entry onto the btree.  In that
** case P2 must be 0.  It is assumed that the cursor is used only for
** appending and so if the cursor is valid, then the cursor must already
** be pointing at the end of the btree and so no changes are made to
** the cursor.
*/
case OP_Last: {        /* jump */
  VdbeCursor *pC;
  BtCursor *pCrsr;
  int res;

  assert( pOp->p1>=0 && pOp->p1<p->nCursor );
  pC = p->apCsr[pOp->p1];
  assert( pC!=0 );
  assert( pC->eCurType==CURTYPE_BTREE );
  pCrsr = pC->uc.pCursor;
  res = 0;
  assert( pCrsr!=0 );




  pC->seekResult = pOp->p3;
#ifdef SQLITE_DEBUG
  pC->seekOp = OP_Last;
#endif
  if( pOp->p3==0 || !sqlite3BtreeCursorIsValidNN(pCrsr) ){
    rc = sqlite3BtreeLast(pCrsr, &res);
    pC->nullRow = (u8)res;
    pC->deferredMoveto = 0;
    pC->cacheStatus = CACHE_STALE;
    if( rc ) goto abort_due_to_error;
    if( pOp->p2>0 ){
      VdbeBranchTaken(res!=0,2);
      if( res ) goto jump_to_p2;
    }
  }else{
    assert( pOp->p2==0 );
  }
  break;
}


/* Opcode: Sort P1 P2 * * *
**