SQLite

      /* A sub-query in the FROM clause of a SELECT */
      assert( pSel!=0 );
      assert( pFrom->pTab==0 );
      if( sqlite3WalkSelect(pWalker, pSel) ) return WRC_Abort;
      pFrom->pTab = pTab = sqlite3DbMallocZero(db, sizeof(Table));
      if( pTab==0 ) return WRC_Abort;
      pTab->nTabRef = 1;
      if( pFrom->zAlias ){
        pTab->zName = sqlite3DbStrDup(db, pFrom->zAlias);
      }else{
        pTab->zName = sqlite3MPrintf(db, "subquery_%p", (void*)pTab);
      }
      while( pSel->pPrior ){ pSel = pSel->pPrior; }
      sqlite3ColumnsFromExprList(pParse, pSel->pEList,&pTab->nCol,&pTab->aCol);
      pTab->iPKey = -1;
      pTab->nRowLogEst = 200; assert( 200==sqlite3LogEst(1048576) );
      pTab->tabFlags |= TF_Ephemeral;
#endif
    }else{

  ** (1) Authorized unreferenced tables
  ** (2) Generate code for all sub-queries
  */
  for(i=0; i<pTabList->nSrc; i++){
    struct SrcList_item *pItem = &pTabList->a[i];
    SelectDest dest;
    Select *pSub;
#if !defined(SQLITE_OMIT_SUBQUERY) || !defined(SQLITE_OMIT_VIEW)
    const char *zSavedAuthContext;
#endif

    /* Issue SQLITE_READ authorizations with a fake column name for any
    ** tables that are referenced but from which no values are extracted.
    ** Examples of where these kinds of null SQLITE_READ authorizations
    ** would occur:
    **
    **     SELECT count(*) FROM t1;   -- SQLITE_READ t1.""

#if SELECTTRACE_ENABLED
      if( sqlite3SelectTrace & 0x100 ){
        SELECTTRACE(0x100,pParse,p,("After WHERE-clause push-down:\n"));
        sqlite3TreeViewSelect(0, p, 0);
      }
#endif
    }

    zSavedAuthContext = pParse->zAuthContext;
    pParse->zAuthContext = pItem->zName;

    /* Generate code to implement the subquery
    **
    ** The subquery is implemented as a co-routine if all of these are true:
    **   (1)  The subquery is guaranteed to be the outer loop (so that it
    **        does not need to be computed more than once)
    **   (2)  REMOVED (2017-09-28): The ALL keyword after SELECT is omitted.

     /*** constraint removed: && (p->selFlags & SF_All)==0             (2) */
     && OptimizationEnabled(db, SQLITE_SubqCoroutine)               /* (3) */
    ){
      /* Implement a co-routine that will return a single row of the result
      ** set on each invocation.
      */
      int addrTop = sqlite3VdbeCurrentAddr(v)+1;
     
      pItem->regReturn = ++pParse->nMem;
      sqlite3VdbeAddOp3(v, OP_InitCoroutine, pItem->regReturn, 0, addrTop);
      VdbeComment((v, "%s", pItem->pTab->zName));
      pItem->addrFillSub = addrTop;
      sqlite3SelectDestInit(&dest, SRT_Coroutine, pItem->regReturn);
      explainSetInteger(pItem->iSelectId, (u8)pParse->iNextSelectId);
      sqlite3Select(pParse, pSub, &dest);

      retAddr = sqlite3VdbeAddOp1(v, OP_Return, pItem->regReturn);
      VdbeComment((v, "end %s", pItem->pTab->zName));
      sqlite3VdbeChangeP1(v, topAddr, retAddr);
      sqlite3ClearTempRegCache(pParse);
    }
    if( db->mallocFailed ) goto select_end;
    pParse->nHeight -= sqlite3SelectExprHeight(p);
    pParse->zAuthContext = zSavedAuthContext;
#endif
  }

  /* Various elements of the SELECT copied into local variables for
  ** convenience */
  pEList = p->pEList;
  pWhere = p->pWhere;

     SELECT * FROM test1 a, test1 b LIMIT 1
  }
} {a.f1 11 a.f2 22 b.f1 11 b.f2 22}
do_test select1-6.9.7 {
  set x [execsql2 {
     SELECT * FROM test1 a, (select 5, 6) LIMIT 1
  }]
  regsub -all {subquery_[0-9a-fA-F_]+} $x {subquery} x
  set x
} {a.f1 11 a.f2 22 subquery.5 5 subquery.6 6}
do_test select1-6.9.8 {
  set x [execsql2 {
     SELECT * FROM test1 a, (select 5 AS x, 6 AS y) AS b LIMIT 1
  }]
  regsub -all {subquery_[0-9a-fA-F]+_} $x {subquery} x
  set x
} {a.f1 11 a.f2 22 b.x 5 b.y 6}