Method optimize

basex-core/src/main/java/org/basex/query/expr/Lookup.java:38–68

(final CompileContext cc)

  }

  @Override
  public Expr optimize(final CompileContext cc) throws QueryException {
    exprs[1] = exprs[1].simplifyFor(Simplify.DATA, cc);

    final Expr inputs = exprs[0];
    final long is = inputs.size();
    if(is == 0) return cc.replaceWith(this, inputs);

    // skip optimizations if input may yield items other than maps or arrays
    final Type tp = inputs.seqType().type;
    final boolean map = tp instanceof MapType, array = tp instanceof ArrayType;
    if(!(map || array)) return this;

    final Expr expr = opt(cc);
    if(expr != this) return cc.replaceWith(this, expr);

    // derive type from input expression
    final Expr keys = exprs[1];
    final SeqType kt = keys.seqType();
    final SeqType st = map ? ((MapType) tp).valueType() : ((ArrayType) tp).valueType();
    Occ occ = st.occ;
    if(inputs.size() != 1 || keys == WILDCARD || !kt.one() || kt.mayBeWrapped()) {
      // key is wildcard, or expressions yield no single item
      occ = occ.union(Occ.ZERO_OR_MORE);
    } else if(map) {
      // map lookup may result in empty sequence
      occ = occ.union(Occ.ZERO);
    }
    exprType.assign(st.type, occ);
    return this;
  }

  /**
   * Rewrites the lookup to another expression.