SQLite

      sqlite3VdbeAddOp3(v, OP_OpenWrite, 1, pParse->regRoot, iDb);
      sqlite3VdbeChangeP5(v, OPFLAG_P2ISREG);
      pParse->nTab = 2;
      addrTop = sqlite3VdbeCurrentAddr(v) + 1;
      sqlite3VdbeAddOp3(v, OP_InitCoroutine, regYield, 0, addrTop);
      sqlite3SelectDestInit(&dest, SRT_Coroutine, regYield);
      sqlite3Select(pParse, pSelect, &dest);
      sqlite3VdbeEndCoroutine(v, regYield);
      sqlite3VdbeJumpHere(v, addrTop - 1);
      if( pParse->nErr ) return;
      pSelTab = sqlite3ResultSetOfSelect(pParse, pSelect);
      if( pSelTab==0 ) return;
      assert( p->aCol==0 );
      p->nCol = pSelTab->nCol;
      p->aCol = pSelTab->aCol;

    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;
    if( rc || db->mallocFailed || pParse->nErr ) goto insert_cleanup;
    sqlite3VdbeEndCoroutine(v, regYield);
    sqlite3VdbeJumpHere(v, addrTop - 1);                       /* label B: */
    assert( pSelect->pEList );
    nColumn = pSelect->pEList->nExpr;

    /* Set useTempTable to TRUE if the result of the SELECT statement
    ** should be written into a temporary table (template 4).  Set to
    ** FALSE if each output row of the SELECT can be written directly into

  */
  addrSelectA = sqlite3VdbeCurrentAddr(v) + 1;
  addr1 = sqlite3VdbeAddOp3(v, OP_InitCoroutine, regAddrA, 0, addrSelectA);
  VdbeComment((v, "left SELECT"));
  pPrior->iLimit = regLimitA;
  explainSetInteger(iSub1, pParse->iNextSelectId);
  sqlite3Select(pParse, pPrior, &destA);
  sqlite3VdbeEndCoroutine(v, regAddrA);
  sqlite3VdbeJumpHere(v, addr1);

  /* Generate a coroutine to evaluate the SELECT statement on 
  ** the right - the "B" select
  */
  addrSelectB = sqlite3VdbeCurrentAddr(v) + 1;
  addr1 = sqlite3VdbeAddOp3(v, OP_InitCoroutine, regAddrB, 0, addrSelectB);
  VdbeComment((v, "right SELECT"));
  savedLimit = p->iLimit;
  savedOffset = p->iOffset;
  p->iLimit = regLimitB;
  p->iOffset = 0;  
  explainSetInteger(iSub2, pParse->iNextSelectId);
  sqlite3Select(pParse, p, &destB);
  p->iLimit = savedLimit;
  p->iOffset = savedOffset;
  sqlite3VdbeEndCoroutine(v, regAddrB);

  /* Generate a subroutine that outputs the current row of the A
  ** select as the next output row of the compound select.
  */
  VdbeNoopComment((v, "Output routine for A"));
  addrOutA = generateOutputSubroutine(pParse,
                 p, &destA, pDest, regOutA,

      pItem->addrFillSub = addrTop;
      sqlite3SelectDestInit(&dest, SRT_Coroutine, pItem->regReturn);
      explainSetInteger(pItem->iSelectId, (u8)pParse->iNextSelectId);
      sqlite3Select(pParse, pSub, &dest);
      pItem->pTab->nRowLogEst = sqlite3LogEst(pSub->nSelectRow);
      pItem->fg.viaCoroutine = 1;
      pItem->regResult = dest.iSdst;
      sqlite3VdbeEndCoroutine(v, pItem->regReturn);
      sqlite3VdbeJumpHere(v, addrTop-1);
      sqlite3ClearTempRegCache(pParse);
    }else{
      /* Generate a subroutine that will fill an ephemeral table with
      ** the content of this subquery.  pItem->addrFillSub will point
      ** to the address of the generated subroutine.  pItem->regReturn
      ** is a register allocated to hold the subroutine return address

int sqlite3VdbeGoto(Vdbe*,int);
int sqlite3VdbeLoadString(Vdbe*,int,const char*);
void sqlite3VdbeMultiLoad(Vdbe*,int,const char*,...);
int sqlite3VdbeAddOp3(Vdbe*,int,int,int,int);
int sqlite3VdbeAddOp4(Vdbe*,int,int,int,int,const char *zP4,int);
int sqlite3VdbeAddOp4Dup8(Vdbe*,int,int,int,int,const u8*,int);
int sqlite3VdbeAddOp4Int(Vdbe*,int,int,int,int,int);
void sqlite3VdbeEndCoroutine(Vdbe*,int);
#if defined(SQLITE_DEBUG) && !defined(SQLITE_TEST_REALLOC_STRESS)
  void sqlite3VdbeVerifyNoMallocRequired(Vdbe *p, int N);
#else
# define sqlite3VdbeVerifyNoMallocRequired(A,B)
#endif
VdbeOp *sqlite3VdbeAddOpList(Vdbe*, int nOp, VdbeOpList const *aOp, int iLineno);
void sqlite3VdbeAddParseSchemaOp(Vdbe*,int,char*);

  int p3,             /* The P3 operand */
  int p4              /* The P4 operand as an integer */
){
  int addr = sqlite3VdbeAddOp3(p, op, p1, p2, p3);
  sqlite3VdbeChangeP4(p, addr, SQLITE_INT_TO_PTR(p4), P4_INT32);
  return addr;
}

/* Insert the end of a co-routine
*/
void sqlite3VdbeEndCoroutine(Vdbe *v, int regYield){
  sqlite3VdbeAddOp1(v, OP_EndCoroutine, regYield);

  /* Clear the temporary register cache, thereby ensuring that each
  ** co-routine has its own independent set of registers, because co-routines
  ** might expect their registers to be preserved across an OP_Yield, and
  ** that could cause problems if two or more co-routines are using the same
  ** temporary register.
  */
  v->pParse->nTempReg = 0;
  v->pParse->nRangeReg = 0;
}

/*
** Create a new symbolic label for an instruction that has yet to be
** coded.  The symbolic label is really just a negative number.  The
** label can be used as the P2 value of an operation.  Later, when
** the label is resolved to a specific address, the VDBE will scan
** through its operation list and change all values of P2 which match

} {123 456}
do_execsql_test select4-14.16 {
  VALUES(1),(2),(3),(4) UNION ALL SELECT 5 LIMIT 99;
} {1 2 3 4 5}
do_execsql_test select4-14.17 {
  VALUES(1),(2),(3),(4) UNION ALL SELECT 5 LIMIT 3;
} {1 2 3}

# Ticket https://www.sqlite.org/src/info/d06a25c84454a372
# Incorrect answer due to two co-routines using the same registers and expecting
# those register values to be preserved across a Yield.
#
do_execsql_test select4-15.1 {
  DROP TABLE IF EXISTS tx;
  CREATE TABLE tx(id INTEGER PRIMARY KEY, a, b);
  INSERT INTO tx(a,b) VALUES(33,456);
  INSERT INTO tx(a,b) VALUES(33,789);

  SELECT DISTINCT t0.id, t0.a, t0.b
    FROM tx AS t0, tx AS t1
   WHERE t0.a=t1.a AND t1.a=33 AND t0.b=456
  UNION
  SELECT DISTINCT t0.id, t0.a, t0.b
    FROM tx AS t0, tx AS t1
   WHERE t0.a=t1.a AND t1.a=33 AND t0.b=789
   ORDER BY 1;
} {1 33 456 2 33 789}

finish_test