SQLite

#define WHERE_FORCE_TABLE      0x0020 /* Do not use an index-only search */
#define WHERE_ONETABLE_ONLY    0x0040 /* Only code the 1st table in pTabList */
#define WHERE_AND_ONLY         0x0080 /* Don't use indices for OR terms */
#define WHERE_GROUPBY          0x0100 /* pOrderBy is really a GROUP BY */
#define WHERE_DISTINCTBY       0x0200 /* pOrderby is really a DISTINCT clause */
#define WHERE_WANT_DISTINCT    0x0400 /* All output needs to be distinct */
#define WHERE_SORTBYGROUP      0x0800 /* Support sqlite3WhereIsSorted() */
#define WHERE_OR_INDEX_OPEN    0x1000 /* OP_OpenRead for the OR index exists */

/* Allowed return values from sqlite3WhereIsDistinct()
*/
#define WHERE_DISTINCT_NOOP      0  /* DISTINCT keyword not used */
#define WHERE_DISTINCT_UNIQUE    1  /* No duplicates */
#define WHERE_DISTINCT_ORDERED   2  /* All duplicates are adjacent */
#define WHERE_DISTINCT_UNORDERED 3  /* Duplicates are scattered */

    int regReturn = ++pParse->nMem;           /* Register used with OP_Gosub */
    int regRowset = 0;                        /* Register for RowSet object */
    int regRowid = 0;                         /* Register holding rowid */
    int iLoopBody = sqlite3VdbeMakeLabel(v);  /* Start of loop body */
    int iRetInit;                             /* Address of regReturn init */
    int untestedTerms = 0;             /* Some terms not completely tested */
    int ii;                            /* Loop counter */
    int subWctrlFlags;                 /* wctrlFlags for sub-queries */
    Expr *pAndExpr = 0;                /* An ".. AND (...)" expression */
    Table *pTab = pTabItem->pTab;
   
    pTerm = pLoop->aLTerm[0];
    assert( pTerm!=0 );
    assert( pTerm->eOperator & WO_OR );
    assert( (pTerm->wtFlags & TERM_ORINFO)!=0 );

      }
    }

    /* Run a separate WHERE clause for each term of the OR clause.  After
    ** eliminating duplicates from other WHERE clauses, the action for each
    ** sub-WHERE clause is to to invoke the main loop body as a subroutine.
    */
    subWctrlFlags = WHERE_OMIT_OPEN_CLOSE | WHERE_AND_ONLY |
                    WHERE_FORCE_TABLE | WHERE_ONETABLE_ONLY;
    for(ii=0; ii<pOrWc->nTerm; ii++){
      WhereTerm *pOrTerm = &pOrWc->a[ii];
      if( pOrTerm->leftCursor==iCur || (pOrTerm->eOperator & WO_AND)!=0 ){
        WhereInfo *pSubWInfo;           /* Info for single OR-term scan */
        Expr *pOrExpr = pOrTerm->pExpr; /* Current OR clause term */
        int j1 = 0;                     /* Address of jump operation */
        if( pAndExpr && !ExprHasProperty(pOrExpr, EP_FromJoin) ){
          pAndExpr->pLeft = pOrExpr;
          pOrExpr = pAndExpr;
        }
        /* Loop through table entries that match term pOrTerm. */
        pSubWInfo = sqlite3WhereBegin(pParse, pOrTab, pOrExpr, 0, 0,
                        WHERE_OMIT_OPEN_CLOSE | WHERE_AND_ONLY |
                        subWctrlFlags, iCovCur);
                        WHERE_FORCE_TABLE | WHERE_ONETABLE_ONLY, iCovCur);
        assert( pSubWInfo || pParse->nErr || db->mallocFailed );
        if( pSubWInfo ){
          WhereLoop *pSubLoop;
          explainOneScan(
              pParse, pOrTab, &pSubWInfo->a[0], iLevel, pLevel->iFrom, 0
          );
          /* This is the sub-WHERE clause body.  First skip over

          assert( (pSubLoop->wsFlags & WHERE_AUTO_INDEX)==0 );
          if( (pSubLoop->wsFlags & WHERE_INDEXED)!=0
           && (ii==0 || pSubLoop->u.btree.pIndex==pCov)
           && (HasRowid(pTab) || !IsPrimaryKeyIndex(pSubLoop->u.btree.pIndex))
          ){
            assert( pSubWInfo->a[0].iIdxCur==iCovCur );
            pCov = pSubLoop->u.btree.pIndex;
            subWctrlFlags |= WHERE_OR_INDEX_OPEN;
          }else{
            pCov = 0;
          }

          /* Finish the loop through table entries that match term pOrTerm. */
          sqlite3WhereEnd(pSubWInfo);
        }

          iIndexCur++;
          pJ = pJ->pNext;
        }
        op = OP_OpenWrite;
        pWInfo->aiCurOnePass[1] = iIndexCur;
      }else if( iIdxCur && (wctrlFlags & WHERE_ONETABLE_ONLY)!=0 ){
        iIndexCur = iIdxCur;
        if( (wctrlFlags & WHERE_OR_INDEX_OPEN)!=0 ){
          /* For 2nd and subsequent subqueries for processing OR terms,
          ** try to reuse the previous OP_OpenRead, if there is one.  The
          ** WHERE_OR_INDEX_OPEN bit will only be set if there is a prior
          ** OP_OpenRead opcode on cursor iIndexCur, so the while() loop
          ** below is guaranteed to terminate.
          */
          VdbeOp *pOp = sqlite3VdbeGetOp(v, -1);
          while( pOp->opcode!=OP_OpenRead || pOp->p1!=iIndexCur ) pOp--;
          assert( pOp->p3==iDb );
          if( pOp->p2==pIx->tnum ) op = 0;
        }
      }else{
        iIndexCur = pParse->nTab++;
      }
      pLevel->iIdxCur = iIndexCur;
      assert( pIx->pSchema==pTab->pSchema );
      assert( iIndexCur>=0 );
      if( op ){

  EXPLAIN QUERY PLAN
    SELECT a.x, b.x
      FROM t12 AS a JOIN t12 AS b ON a.y=b.x
     WHERE (b.x=$abc OR b.y=$abc);
} {/.*SEARCH TABLE t12 AS b .*SEARCH TABLE t12 AS b .*/}
}

# Verify that all necessary OP_OpenRead opcodes occur in the OR optimization.
#
do_execsql_test where2-13.1 {
  CREATE TABLE t13(a,b);
  CREATE INDEX t13a ON t13(a);
  INSERT INTO t13 VALUES(4,5);
  SELECT * FROM t13 WHERE (1=2 AND a=3) OR a=4;
} {4 5}

finish_test