SQLite

static void explain_data_prepare(ShellState *p, sqlite3_stmt *pSql){
  const char *zSql;               /* The text of the SQL statement */
  const char *z;                  /* Used to check if this is an EXPLAIN */
  int *abYield = 0;               /* True if op is an OP_Yield */
  int nAlloc = 0;                 /* Allocated size of p->aiIndent[], abYield */
  int iOp;                        /* Index of operation in p->aiIndent[] */

  const char *azNext[] = { "Next", "Prev", "VPrev", "VNext", "SorterNext",
                           "NextIfOpen", "PrevIfOpen", 0 };
  const char *azYield[] = { "Yield", "SeekLT", "SeekGT", "RowSetRead",
                            "Rewind", 0 };
  const char *azGoto[] = { "Goto", 0 };

  /* Try to figure out if this is really an EXPLAIN statement. If this
  ** cannot be verified, return early.  */
  if( sqlite3_column_count(pSql)!=8 ){

**
** P4 is always of type P4_ADVANCE. The function pointer points to
** sqlite3BtreeNext().
**
** If P5 is positive and the jump is taken, then event counter
** number P5-1 in the prepared statement is incremented.
**
** See also: Prev, NextIfOpen
*/
/* Opcode: NextIfOpen P1 P2 P3 P4 P5
**
** This opcode works just like Next except that if cursor P1 is not
** open it behaves a no-op.
*/
/* Opcode: Prev P1 P2 P3 P4 P5
**
** Back up cursor P1 so that it points to the previous key/data pair in its
** table or index.  If there is no previous key/value pairs then fall through
** to the following instruction.  But if the cursor backup was successful,
** jump immediately to P2.

**
** P4 is always of type P4_ADVANCE. The function pointer points to
** sqlite3BtreePrevious().
**
** If P5 is positive and the jump is taken, then event counter
** number P5-1 in the prepared statement is incremented.
*/
/* Opcode: PrevIfOpen P1 P2 P3 P4 P5
**
** This opcode works just like Prev except that if cursor P1 is not
** open it behaves a no-op.
*/
/* Opcode: SorterNext P1 P2 * * P5
**
** This opcode works just like OP_Next except that P1 must be a
** sorter object for which the OP_SorterSort opcode has been
** invoked.  This opcode advances the cursor to the next sorted
** record, or jumps to P2 if there are no more sorted records.
*/
case OP_SorterNext: {  /* jump */
  VdbeCursor *pC;

  pC = p->apCsr[pOp->p1];
  assert( isSorter(pC) );
  rc = sqlite3VdbeSorterNext(db, pC);
  goto next_tail;
case OP_PrevIfOpen:    /* jump */
case OP_NextIfOpen:    /* jump */
  if( p->apCsr[pOp->p1]==0 ) break;
  /* Fall through */
case OP_Prev:          /* jump */
case OP_Next:          /* jump */
  assert( pOp->p1>=0 && pOp->p1<p->nCursor );
  assert( pOp->p5<ArraySize(p->aCounter) );
  pC = p->apCsr[pOp->p1];
  assert( pC!=0 );
  assert( pC->deferredMoveto==0 );
  assert( pC->eCurType==CURTYPE_BTREE );
  assert( pOp->opcode!=OP_Next || pOp->p4.xAdvance==sqlite3BtreeNext );
  assert( pOp->opcode!=OP_Prev || pOp->p4.xAdvance==sqlite3BtreePrevious );
  assert( pOp->opcode!=OP_NextIfOpen || pOp->p4.xAdvance==sqlite3BtreeNext );
  assert( pOp->opcode!=OP_PrevIfOpen || pOp->p4.xAdvance==sqlite3BtreePrevious);


  /* The Next opcode is only used after SeekGT, SeekGE, and Rewind.
  ** The Prev opcode is only used after SeekLT, SeekLE, and Last. */
  assert( pOp->opcode!=OP_Next || pOp->opcode!=OP_NextIfOpen
       || pC->seekOp==OP_SeekGT || pC->seekOp==OP_SeekGE
       || pC->seekOp==OP_Rewind || pC->seekOp==OP_Found);
  assert( pOp->opcode!=OP_Prev || pOp->opcode!=OP_PrevIfOpen
       || pC->seekOp==OP_SeekLT || pC->seekOp==OP_SeekLE
       || pC->seekOp==OP_Last );


  rc = pOp->p4.xAdvance(pC->uc.pCursor, pOp->p3);
next_tail:
  pC->cacheStatus = CACHE_STALE;
  VdbeBranchTaken(rc==SQLITE_OK,2);
  if( rc==SQLITE_OK ){
    pC->nullRow = 0;

        case OP_Vacuum:
        case OP_JournalMode: {
          p->readOnly = 0;
          p->bIsReader = 1;
          break;
        }
        case OP_Next:
        case OP_NextIfOpen:
        case OP_SorterNext: {
          pOp->p4.xAdvance = sqlite3BtreeNext;
          pOp->p4type = P4_ADVANCE;
          /* The code generator never codes any of these opcodes as a jump
          ** to a label.  They are always coded as a jump backwards to a 
          ** known address */
          assert( pOp->p2>=0 );
          break;
        }
        case OP_Prev:
        case OP_PrevIfOpen: {
          pOp->p4.xAdvance = sqlite3BtreePrevious;
          pOp->p4type = P4_ADVANCE;
          /* The code generator never codes any of these opcodes as a jump
          ** to a label.  They are always coded as a jump backwards to a 
          ** known address */
          assert( pOp->p2>=0 );
          break;

            sqlite3VdbeAddOp4Int(v, OP_IfNoHope, pLevel->iIdxCur,
                              sqlite3VdbeCurrentAddr(v)+2,
                              pIn->iBase, pIn->nPrefix);
            VdbeCoverage(v);
          }
          sqlite3VdbeAddOp2(v, pIn->eEndLoopOp, pIn->iCur, pIn->addrInTop);
          VdbeCoverage(v);
          VdbeCoverageIf(v, pIn->eEndLoopOp==OP_PrevIfOpen);
          VdbeCoverageIf(v, pIn->eEndLoopOp==OP_NextIfOpen);
        }
        sqlite3VdbeJumpHere(v, pIn->addrInTop-1);
      }
    }
    sqlite3VdbeResolveLabel(v, pLevel->addrBrk);
    if( pLevel->addrSkip ){
      sqlite3VdbeGoto(v, pLevel->addrSkip);

          }else{
            int iCol = aiMap ? aiMap[iMap++] : 0;
            pIn->addrInTop = sqlite3VdbeAddOp3(v,OP_Column,iTab, iCol, iOut);
          }
          sqlite3VdbeAddOp1(v, OP_IsNull, iOut); VdbeCoverage(v);
          if( i==iEq ){
            pIn->iCur = iTab;
            pIn->eEndLoopOp = bRev ? OP_PrevIfOpen : OP_NextIfOpen;
            if( iEq>0 && (pLoop->wsFlags & WHERE_VIRTUALTABLE)==0 ){
              pIn->iBase = iReg - i;
              pIn->nPrefix = i;
              pLoop->wsFlags |= WHERE_IN_EARLYOUT;
            }else{
              pIn->nPrefix = 0;
            }