Github user fhueske commented on a diff in the pull request:
https://github.com/apache/flink/pull/3520#discussion_r106135951
--- Diff:
flink-libraries/flink-table/src/main/scala/org/apache/flink/table/sources/FilterableTableSource.scala
---
@@ -0,0 +1,66 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.table.sources
+
+import org.apache.flink.table.expressions.Expression
+
+/**
+ * Adds support for filtering push-down to a [[TableSource]].
+ * A [[TableSource]] extending this interface is able to filter records
before returning.
+ */
+trait FilterableTableSource[T] extends TableSource[T] {
+
+ /**
+ * Indicates whether the filter push down has been applied. Note that
even if we don't
+ * actually push down any filters, we should also set this flag to true
after the trying.
+ */
+ private var filterPushedDown: Boolean = false
+
+ /**
+ * Check and pick all predicates this table source can support. The
passed in predicates
+ * have been translated in conjunctive form, and table source can only
pick those predicates
+ * that it supports.
+ * <p>
+ * After trying to push predicates down, we should return a new
[[FilterableTableSource]]
+ * instance which holds all pushed down predicates. Even if we actually
pushed nothing down,
+ * it is recommended that we still return a new
[[FilterableTableSource]] instance since we will
+ * mark the returned instance as filter push down has been tried. Also
we need to return all
+ * unsupported predicates back to the framework to do further filtering.
+ * <p>
+ * We also should note to not changing the form of the predicates
passed in. It has been
+ * organized in CNF conjunctive form, and we should only take or leave
each element in the
+ * array. Don't try to reorganize the predicates if you are absolutely
confident with that.
+ *
+ * @param predicate An array contains conjunctive predicates.
+ * @return A new cloned instance of [[FilterableTableSource]] as well
as n array of Expression
+ * which contains all unsupported predicates.
+ */
+ def applyPredicate(predicate: Array[Expression]):
(FilterableTableSource[_], Array[Expression])
--- End diff --
Maybe should design this as:
```
// returns a copy of the table source with pushed down predicates
applyPredicate(predicate: Array[Expression]): FilterableTableSource[_]
// returns pushed down predicates
getPredicates(): Array[Expression]
```
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