Java Code Examples for org.apache.calcite.util.mapping.Mappings#create()

@Override
public void onMatch(RelOptRuleCall call) {
  FilterPrel filter = call.rel(0);
  NestedLoopJoinPrel join = call.rel(1);

  if ((join.getProjectedFields() == null) || join.getProjectedFields().cardinality() == join.getInputRowType().getFieldCount()) {
    call.transformTo(NestedLoopJoinPrel.create(join.getCluster(), join.getTraitSet(), join.getLeft(), join.getRight(), join.getJoinType(), RelOptUtil.andJoinFilters(join.getCluster().getRexBuilder(), join.getCondition(), filter.getCondition()), join.getProjectedFields()));
  } else {
    // Current filter condition is written based on projected fields on join. In order to push this filter down we need to rewrite filter condition
    final ImmutableBitSet topProjectedColumns = RelOptUtil.InputFinder.bits(filter.getCondition());
    final ImmutableBitSet bottomProjectedColumns = join.getProjectedFields();

    Mapping mapping = Mappings.create(MappingType.SURJECTION, join.getRowType().getFieldCount(), join.getInputRowType().getFieldCount());
    for (Ord<Integer> ord : Ord.zip(bottomProjectedColumns)) {
      if (topProjectedColumns.get(ord.i)) {
        mapping.set(ord.i, ord.e);
      }
    }

    RexShuttle shuttle = new RexPermuteInputsShuttle(mapping);
    RexNode updatedCondition = shuttle.apply(filter.getCondition());

    call.transformTo(NestedLoopJoinPrel.create(join.getCluster(), join.getTraitSet(), join.getLeft(), join.getRight(), join.getJoinType(), RelOptUtil.andJoinFilters(join.getCluster().getRexBuilder(), join.getCondition(), updatedCondition), join.getProjectedFields()));
  }
}
 

/**
 * Returns a permutation describing where output fields come from. In
 * the returned map, value of {@code map.getTargetOpt(i)} is {@code n} if
 * field {@code i} projects input field {@code n} or applies a cast on
 * {@code n}, -1 if it is another expression.
 */
public static Mappings.TargetMapping permutationIgnoreCast(
    List<RexNode> nodes,
    RelDataType inputRowType) {
  final Mappings.TargetMapping mapping =
      Mappings.create(
          MappingType.PARTIAL_FUNCTION,
          nodes.size(),
          inputRowType.getFieldCount());
  for (Ord<RexNode> node : Ord.zip(nodes)) {
    if (node.e instanceof RexInputRef) {
      mapping.set(
          node.i,
          ((RexInputRef) node.e).getIndex());
    } else if (node.e.isA(SqlKind.CAST)) {
      final RexNode operand = ((RexCall) node.e).getOperands().get(0);
      if (operand instanceof RexInputRef) {
        mapping.set(node.i, ((RexInputRef) operand).getIndex());
      }
    }
  }
  return mapping;
}
 

/**
 * Returns a mapping of a set of project expressions.
 *
 * <p>The mapping is an inverse surjection.
 * Every target has a source field, but no
 * source has more than one target.
 * Thus you can safely call
 * {@link org.apache.calcite.util.mapping.Mappings.TargetMapping#getSourceOpt(int)}.
 *
 * @param inputFieldCount Number of input fields
 * @param projects Project expressions
 * @return Mapping of a set of project expressions, or null if projection is
 * not a mapping
 */
public static Mappings.TargetMapping getMapping(int inputFieldCount,
    List<? extends RexNode> projects) {
  if (inputFieldCount < projects.size()) {
    return null; // surjection is not possible
  }
  Mappings.TargetMapping mapping =
      Mappings.create(MappingType.INVERSE_SURJECTION,
          inputFieldCount, projects.size());
  for (Ord<RexNode> exp : Ord.<RexNode>zip(projects)) {
    if (!(exp.e instanceof RexInputRef)) {
      return null;
    }

    int source = ((RexInputRef) exp.e).getIndex();
    if (mapping.getTargetOpt(source) != -1) {
      return null;
    }
    mapping.set(source, exp.i);
  }
  return mapping;
}
 

/**
 * Returns a permutation describing where output fields come from. In
 * the returned map, value of {@code map.getTargetOpt(i)} is {@code n} if
 * field {@code i} projects input field {@code n}, -1 if it is an
 * expression.
 */
public static Mappings.TargetMapping permutation(
    List<RexNode> nodes,
    RelDataType inputRowType) {
  final Mappings.TargetMapping mapping =
      Mappings.create(
          MappingType.PARTIAL_FUNCTION,
          nodes.size(),
          inputRowType.getFieldCount());
  for (Ord<RexNode> node : Ord.zip(nodes)) {
    if (node.e instanceof RexInputRef) {
      mapping.set(
          node.i,
          ((RexInputRef) node.e).getIndex());
    }
  }
  return mapping;
}
 

/**
 * Returns a permutation describing where output fields come from. In
 * the returned map, value of {@code map.getTargetOpt(i)} is {@code n} if
 * field {@code i} projects input field {@code n} or applies a cast on
 * {@code n}, -1 if it is another expression.
 */
public static Mappings.TargetMapping permutationIgnoreCast(List<RexNode> nodes, RelDataType inputRowType) {
    final Mappings.TargetMapping mapping = Mappings.create(MappingType.PARTIAL_FUNCTION, nodes.size(),
            inputRowType.getFieldCount());
    for (Ord<RexNode> node : Ord.zip(nodes)) {
        if (node.e instanceof RexInputRef) {
            mapping.set(node.i, ((RexInputRef) node.e).getIndex());
        } else if (node.e.isA(SqlKind.CAST)) {
            final RexNode operand = ((RexCall) node.e).getOperands().get(0);
            if (operand instanceof RexInputRef) {
                mapping.set(node.i, ((RexInputRef) operand).getIndex());
            }
        }
    }
    return mapping;
}
 

/**
 * Returns a mapping of a set of project expressions.
 *
 * <p>The mapping is an inverse surjection.
 * Every target has a source field, but
 * a source field may appear as zero, one, or more target fields.
 * Thus you can safely call
 * {@link org.apache.calcite.util.mapping.Mappings.TargetMapping#getTarget(int)}.
 *
 * @param inputFieldCount Number of input fields
 * @param projects Project expressions
 * @return Mapping of a set of project expressions, or null if projection is
 * not a mapping
 */
public static Mappings.TargetMapping getMapping(int inputFieldCount,
    List<? extends RexNode> projects) {
  if (inputFieldCount < projects.size()) {
    return null; // surjection is not possible
  }
  Mappings.TargetMapping mapping =
      Mappings.create(MappingType.INVERSE_SURJECTION,
          inputFieldCount, projects.size());
  for (Ord<RexNode> exp : Ord.<RexNode>zip(projects)) {
    if (!(exp.e instanceof RexInputRef)) {
      return null;
    }
    mapping.set(((RexInputRef) exp.e).getIndex(), exp.i);
  }
  return mapping;
}
 

/**
 * Returns a partial mapping of a set of project expressions.
 *
 * <p>The mapping is an inverse function.
 * Every target has a source field, but
 * a source might have 0, 1 or more targets.
 * Project expressions that do not consist of
 * a mapping are ignored.
 *
 * @param inputFieldCount Number of input fields
 * @return Mapping of a set of project expressions, never null
 */
public Mappings.TargetMapping getPartialMapping(int inputFieldCount) {
  Mappings.TargetMapping mapping =
      Mappings.create(MappingType.INVERSE_FUNCTION,
          inputFieldCount, projects.size());
  for (Ord<RexLocalRef> exp : Ord.zip(projects)) {
    RexNode rexNode = expandLocalRef(exp.e);
    if (rexNode instanceof RexInputRef) {
      mapping.set(((RexInputRef) rexNode).getIndex(), exp.i);
    }
  }
  return mapping;
}
 

protected Mapping createMapping(ImmutableBitSet fieldsUsed, int fieldCount) {
    final Mapping mapping = Mappings.create(MappingType.INVERSE_SURJECTION, fieldCount, fieldsUsed.cardinality());
    int i = 0;
    for (int field : fieldsUsed) {
        mapping.set(field, i++);
    }
    return mapping;
}
 

/**
 * Returns a permutation describing where output fields come from. In
 * the returned map, value of {@code map.getTargetOpt(i)} is {@code n} if
 * field {@code i} projects input field {@code n}, -1 if it is an
 * expression.
 */
public static Mappings.TargetMapping permutation(List<RexNode> nodes, RelDataType inputRowType) {
    final Mappings.TargetMapping mapping = Mappings.create(MappingType.PARTIAL_FUNCTION, nodes.size(),
            inputRowType.getFieldCount());
    for (Ord<RexNode> node : Ord.zip(nodes)) {
        if (node.e instanceof RexInputRef) {
            mapping.set(node.i, ((RexInputRef) node.e).getIndex());
        }
    }
    return mapping;
}
 

/**
 * Extract projects from the Aggregate and return the index mapping between the new projects
 * and it's input.
 */
private Mapping extractProjectsAndMapping(
	Aggregate aggregate,
	RelNode input,
	RelBuilder relBuilder) {

	// Compute which input fields are used.
	final ImmutableBitSet.Builder inputFieldsUsed = getInputFieldUsed(aggregate, input);

	final List<RexNode> projects = new ArrayList<>();
	final Mapping mapping =
		Mappings.create(MappingType.INVERSE_SURJECTION,
			aggregate.getInput().getRowType().getFieldCount(),
			inputFieldsUsed.cardinality());
	int j = 0;
	for (int i : inputFieldsUsed.build()) {
		projects.add(relBuilder.field(i));
		mapping.set(i, j++);
	}

	if (input instanceof Project) {
		// this will not create trivial projects
		relBuilder.project(projects);
	} else {
		relBuilder.project(projects, Collections.emptyList(), true);
	}

	return mapping;
}
 

/**
 * Variant of {@link #trimFields(RelNode, ImmutableBitSet, Set)} for {@link ScanCrel}.
 */
@SuppressWarnings("unused")
public TrimResult trimFields(
    ScanCrel crel,
    ImmutableBitSet fieldsUsed,
    Set<RelDataTypeField> extraFields) {

  if(fieldsUsed.cardinality() == crel.getRowType().getFieldCount()) {
    return result(crel, Mappings.createIdentity(crel.getRowType().getFieldCount()));
  }

  if(fieldsUsed.cardinality() == 0) {
    // do something similar to dummy project but avoid using a scan field. This ensures the scan
    // does a skipAll operation rather than projectin a useless column.
    final RelOptCluster cluster = crel.getCluster();
    final Mapping mapping = Mappings.create(MappingType.INVERSE_SURJECTION, crel.getRowType().getFieldCount(), 1);
    final RexLiteral expr = cluster.getRexBuilder().makeExactLiteral(BigDecimal.ZERO);
    builder.push(crel);
    builder.project(ImmutableList.<RexNode>of(expr), ImmutableList.of("DUMMY"));
    return result(builder.build(), mapping);
  }

  final List<SchemaPath> paths = new ArrayList<>();
  final Mapping m = Mappings.create(MappingType.PARTIAL_FUNCTION, crel.getRowType().getFieldCount(), fieldsUsed.cardinality());
  int index = 0;
  for(int i : fieldsUsed) {
    paths.add(SchemaPath.getSimplePath(crel.getRowType().getFieldList().get(i).getName()));
    m.set(i, index);
    index++;
  }

  ScanCrel newCrel = crel.cloneWithProject(paths);
  return result(newCrel, m);
}
 

/**
 * Extract projects from the Aggregate and return the index mapping between the new projects
 * and it's input.
 */
private Mapping extractProjectsAndMapping(
	Aggregate aggregate,
	RelNode input,
	RelBuilder relBuilder) {

	// Compute which input fields are used.
	final ImmutableBitSet.Builder inputFieldsUsed = getInputFieldUsed(aggregate, input);

	final List<RexNode> projects = new ArrayList<>();
	final Mapping mapping =
		Mappings.create(MappingType.INVERSE_SURJECTION,
			aggregate.getInput().getRowType().getFieldCount(),
			inputFieldsUsed.cardinality());
	int j = 0;
	for (int i : inputFieldsUsed.build()) {
		projects.add(relBuilder.field(i));
		mapping.set(i, j++);
	}

	if (input instanceof Project) {
		// this will not create trivial projects
		relBuilder.project(projects);
	} else {
		relBuilder.project(projects, Collections.emptyList(), true);
	}

	return mapping;
}
 

/** Equivalent to
 * {@link RelOptUtil#createProject(org.apache.calcite.rel.RelNode, java.util.List)}
 * for {@link MutableRel}. */
public static MutableRel createProject(final MutableRel child,
    final List<Integer> posList) {
  final RelDataType rowType = child.rowType;
  if (Mappings.isIdentity(posList, rowType.getFieldCount())) {
    return child;
  }
  final Mapping mapping =
      Mappings.create(
          MappingType.INVERSE_SURJECTION,
          rowType.getFieldCount(),
          posList.size());
  for (int i = 0; i < posList.size(); i++) {
    mapping.set(posList.get(i), i);
  }
  return MutableProject.of(
      RelOptUtil.permute(child.cluster.getTypeFactory(), rowType, mapping),
      child,
      new AbstractList<RexNode>() {
        public int size() {
          return posList.size();
        }

        public RexNode get(int index) {
          final int pos = posList.get(index);
          return RexInputRef.of(pos, rowType);
        }
      });
}
 

/**
 * Infers predicates for an Aggregate.
 *
 * <p>Pulls up predicates that only contains references to columns in the
 * GroupSet. For e.g.
 *
 * <blockquote><pre>
 * inputPullUpExprs : { a &gt; 7, b + c &lt; 10, a + e = 9}
 * groupSet         : { a, b}
 * pulledUpExprs    : { a &gt; 7}
 * </pre></blockquote>
 */
public RelOptPredicateList getPredicates(Aggregate agg, RelMetadataQuery mq) {
  final RelNode input = agg.getInput();
  final RexBuilder rexBuilder = agg.getCluster().getRexBuilder();
  final RelOptPredicateList inputInfo = mq.getPulledUpPredicates(input);
  final List<RexNode> aggPullUpPredicates = new ArrayList<>();

  ImmutableBitSet groupKeys = agg.getGroupSet();
  if (groupKeys.isEmpty()) {
    // "GROUP BY ()" can convert an empty relation to a non-empty relation, so
    // it is not valid to pull up predicates. In particular, consider the
    // predicate "false": it is valid on all input rows (trivially - there are
    // no rows!) but not on the output (there is one row).
    return RelOptPredicateList.EMPTY;
  }
  Mapping m = Mappings.create(MappingType.PARTIAL_FUNCTION,
      input.getRowType().getFieldCount(), agg.getRowType().getFieldCount());

  int i = 0;
  for (int j : groupKeys) {
    m.set(j, i++);
  }

  for (RexNode r : inputInfo.pulledUpPredicates) {
    ImmutableBitSet rCols = RelOptUtil.InputFinder.bits(r);
    if (groupKeys.contains(rCols)) {
      r = r.accept(new RexPermuteInputsShuttle(m, input));
      aggPullUpPredicates.add(r);
    }
  }
  return RelOptPredicateList.of(rexBuilder, aggPullUpPredicates);
}
 

protected Mapping createMapping(ImmutableBitSet fieldsUsed, int fieldCount) {
  final Mapping mapping =
      Mappings.create(
          MappingType.INVERSE_SURJECTION,
          fieldCount,
          fieldsUsed.cardinality());
  int i = 0;
  for (int field : fieldsUsed) {
    mapping.set(field, i++);
  }
  return mapping;
}
 

/**
 * Infers predicates for an Aggregate.
 *
 * <p>Pulls up predicates that only contains references to columns in the
 * GroupSet. For e.g.
 *
 * <blockquote><pre>
 * inputPullUpExprs : { a &gt; 7, b + c &lt; 10, a + e = 9}
 * groupSet         : { a, b}
 * pulledUpExprs    : { a &gt; 7}
 * </pre></blockquote>
 */
public RelOptPredicateList getPredicates(Aggregate agg, RelMetadataQuery mq) {
  final RelNode input = agg.getInput();
  final RexBuilder rexBuilder = agg.getCluster().getRexBuilder();
  final RelOptPredicateList inputInfo = mq.getPulledUpPredicates(input);
  final List<RexNode> aggPullUpPredicates = new ArrayList<>();

  ImmutableBitSet groupKeys = agg.getGroupSet();
  if (groupKeys.isEmpty()) {
    // "GROUP BY ()" can convert an empty relation to a non-empty relation, so
    // it is not valid to pull up predicates. In particular, consider the
    // predicate "false": it is valid on all input rows (trivially - there are
    // no rows!) but not on the output (there is one row).
    return RelOptPredicateList.EMPTY;
  }
  Mapping m = Mappings.create(MappingType.PARTIAL_FUNCTION,
      input.getRowType().getFieldCount(), agg.getRowType().getFieldCount());

  int i = 0;
  for (int j : groupKeys) {
    m.set(j, i++);
  }

  for (RexNode r : inputInfo.pulledUpPredicates) {
    ImmutableBitSet rCols = RelOptUtil.InputFinder.bits(r);
    if (groupKeys.contains(rCols)) {
      r = r.accept(new RexPermuteInputsShuttle(m, input));
      aggPullUpPredicates.add(r);
    }
  }
  return RelOptPredicateList.of(rexBuilder, aggPullUpPredicates);
}
 

/**
 * Infers predicates for a project.
 *
 * <ol>
 * <li>create a mapping from input to projection. Map only positions that
 * directly reference an input column.
 * <li>Expressions that only contain above columns are retained in the
 * Project's pullExpressions list.
 * <li>For e.g. expression 'a + e = 9' below will not be pulled up because 'e'
 * is not in the projection list.
 *
 * <blockquote><pre>
 * inputPullUpExprs:      {a &gt; 7, b + c &lt; 10, a + e = 9}
 * projectionExprs:       {a, b, c, e / 2}
 * projectionPullupExprs: {a &gt; 7, b + c &lt; 10}
 * </pre></blockquote>
 *
 * </ol>
 */
public RelOptPredicateList getPredicates(Project project,
    RelMetadataQuery mq) {
  final RelNode input = project.getInput();
  final RexBuilder rexBuilder = project.getCluster().getRexBuilder();
  final RelOptPredicateList inputInfo = mq.getPulledUpPredicates(input);
  final List<RexNode> projectPullUpPredicates = new ArrayList<>();

  ImmutableBitSet.Builder columnsMappedBuilder = ImmutableBitSet.builder();
  Mapping m = Mappings.create(MappingType.PARTIAL_FUNCTION,
      input.getRowType().getFieldCount(),
      project.getRowType().getFieldCount());

  for (Ord<RexNode> expr : Ord.zip(project.getProjects())) {
    if (expr.e instanceof RexInputRef) {
      int sIdx = ((RexInputRef) expr.e).getIndex();
      m.set(sIdx, expr.i);
      columnsMappedBuilder.set(sIdx);
    // Project can also generate constants. We need to include them.
    } else if (RexLiteral.isNullLiteral(expr.e)) {
      projectPullUpPredicates.add(
          rexBuilder.makeCall(SqlStdOperatorTable.IS_NULL,
              rexBuilder.makeInputRef(project, expr.i)));
    } else if (RexUtil.isConstant(expr.e)) {
      final List<RexNode> args =
          ImmutableList.of(rexBuilder.makeInputRef(project, expr.i), expr.e);
      final SqlOperator op = args.get(0).getType().isNullable()
          || args.get(1).getType().isNullable()
          ? SqlStdOperatorTable.IS_NOT_DISTINCT_FROM
          : SqlStdOperatorTable.EQUALS;
      projectPullUpPredicates.add(rexBuilder.makeCall(op, args));
    }
  }

  // Go over childPullUpPredicates. If a predicate only contains columns in
  // 'columnsMapped' construct a new predicate based on mapping.
  final ImmutableBitSet columnsMapped = columnsMappedBuilder.build();
  for (RexNode r : inputInfo.pulledUpPredicates) {
    RexNode r2 = projectPredicate(rexBuilder, input, r, columnsMapped);
    if (!r2.isAlwaysTrue()) {
      r2 = r2.accept(new RexPermuteInputsShuttle(m, input));
      projectPullUpPredicates.add(r2);
    }
  }
  return RelOptPredicateList.of(rexBuilder, projectPullUpPredicates);
}
 

/**
 * Infers predicates for a project.
 *
 * <ol>
 * <li>create a mapping from input to projection. Map only positions that
 * directly reference an input column.
 * <li>Expressions that only contain above columns are retained in the
 * Project's pullExpressions list.
 * <li>For e.g. expression 'a + e = 9' below will not be pulled up because 'e'
 * is not in the projection list.
 *
 * <blockquote><pre>
 * inputPullUpExprs:      {a &gt; 7, b + c &lt; 10, a + e = 9}
 * projectionExprs:       {a, b, c, e / 2}
 * projectionPullupExprs: {a &gt; 7, b + c &lt; 10}
 * </pre></blockquote>
 *
 * </ol>
 */
public RelOptPredicateList getPredicates(Project project,
    RelMetadataQuery mq) {
  final RelNode input = project.getInput();
  final RexBuilder rexBuilder = project.getCluster().getRexBuilder();
  final RelOptPredicateList inputInfo = mq.getPulledUpPredicates(input);
  final List<RexNode> projectPullUpPredicates = new ArrayList<>();

  ImmutableBitSet.Builder columnsMappedBuilder = ImmutableBitSet.builder();
  Mapping m = Mappings.create(MappingType.PARTIAL_FUNCTION,
      input.getRowType().getFieldCount(),
      project.getRowType().getFieldCount());

  for (Ord<RexNode> expr : Ord.zip(project.getProjects())) {
    if (expr.e instanceof RexInputRef) {
      int sIdx = ((RexInputRef) expr.e).getIndex();
      m.set(sIdx, expr.i);
      columnsMappedBuilder.set(sIdx);
    // Project can also generate constants. We need to include them.
    } else if (RexLiteral.isNullLiteral(expr.e)) {
      projectPullUpPredicates.add(
          rexBuilder.makeCall(SqlStdOperatorTable.IS_NULL,
              rexBuilder.makeInputRef(project, expr.i)));
    } else if (RexUtil.isConstant(expr.e)) {
      final List<RexNode> args =
          ImmutableList.of(rexBuilder.makeInputRef(project, expr.i), expr.e);
      final SqlOperator op = args.get(0).getType().isNullable()
          || args.get(1).getType().isNullable()
          ? SqlStdOperatorTable.IS_NOT_DISTINCT_FROM
          : SqlStdOperatorTable.EQUALS;
      projectPullUpPredicates.add(rexBuilder.makeCall(op, args));
    }
  }

  // Go over childPullUpPredicates. If a predicate only contains columns in
  // 'columnsMapped' construct a new predicate based on mapping.
  final ImmutableBitSet columnsMapped = columnsMappedBuilder.build();
  for (RexNode r : inputInfo.pulledUpPredicates) {
    RexNode r2 = projectPredicate(rexBuilder, input, r, columnsMapped);
    if (!r2.isAlwaysTrue()) {
      r2 = r2.accept(new RexPermuteInputsShuttle(m, input));
      projectPullUpPredicates.add(r2);
    }
  }
  return RelOptPredicateList.of(rexBuilder, projectPullUpPredicates);
}
 

static Pair<RelTraitSet, List<RelTraitSet>> deriveTraitsForProject(
    RelTraitSet childTraits, int childId, List<RexNode> exps,
    RelDataType inputRowType, RelDataTypeFactory typeFactory, RelTraitSet currentTraits) {
  final RelCollation collation = childTraits.getCollation();
  if (collation == null || collation == RelCollations.EMPTY) {
    return null;
  }

  final int maxField = Math.max(exps.size(),
      inputRowType.getFieldCount());
  Mappings.TargetMapping mapping = Mappings
      .create(MappingType.FUNCTION, maxField, maxField);
  for (Ord<RexNode> node : Ord.zip(exps)) {
    if (node.e instanceof RexInputRef) {
      mapping.set(((RexInputRef) node.e).getIndex(), node.i);
    } else if (node.e.isA(SqlKind.CAST)) {
      final RexNode operand = ((RexCall) node.e).getOperands().get(0);
      if (operand instanceof RexInputRef) {
        mapping.set(((RexInputRef) operand).getIndex(), node.i);
      }
    }
  }

  List<RelFieldCollation> collationFieldsToDerive = new ArrayList<>();
  for (RelFieldCollation rc : collation.getFieldCollations()) {
    if (isCollationOnTrivialExpr(exps, typeFactory, mapping, rc, false)) {
      collationFieldsToDerive.add(rc);
    } else {
      break;
    }
  }

  if (collationFieldsToDerive.size() > 0) {
    final RelCollation newCollation = RelCollations
        .of(collationFieldsToDerive).apply(mapping);
    return Pair.of(currentTraits.replace(newCollation),
        ImmutableList.of(currentTraits.replace(collation)));
  } else {
    return null;
  }
}
 

/**
 * Returns a partial mapping of a set of project expressions.
 *
 * <p>The mapping is an inverse function.
 * Every target has a source field, but
 * a source might have 0, 1 or more targets.
 * Project expressions that do not consist of
 * a mapping are ignored.
 *
 * @param inputFieldCount Number of input fields
 * @param projects Project expressions
 * @return Mapping of a set of project expressions, never null
 */
public static Mappings.TargetMapping getPartialMapping(int inputFieldCount,
    List<? extends RexNode> projects) {
  Mappings.TargetMapping mapping =
      Mappings.create(MappingType.INVERSE_FUNCTION,
          inputFieldCount, projects.size());
  for (Ord<RexNode> exp : Ord.<RexNode>zip(projects)) {
    if (exp.e instanceof RexInputRef) {
      mapping.set(((RexInputRef) exp.e).getIndex(), exp.i);
    }
  }
  return mapping;
}