Github user fhueske commented on a diff in the pull request:
https://github.com/apache/flink/pull/3406#discussion_r104177526
--- Diff:
flink-libraries/flink-table/src/main/scala/org/apache/flink/table/catalog/ExternalCatalogSchema.scala
---
@@ -0,0 +1,166 @@
+/*
+ * 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.catalog
+
+import java.util
+
+import org.apache.calcite.linq4j.tree.Expression
+import org.apache.calcite.schema._
+import org.apache.flink.table.api.{DatabaseNotExistException,
TableNotExistException}
+import org.slf4j.{Logger, LoggerFactory}
+
+import scala.collection.JavaConverters._
+
+/**
+ * This class is responsible for connect external catalog to calcite
catalog.
+ * In this way, it is possible to look-up and access tables in SQL queries
+ * without registering tables in advance.
+ * The databases in the external catalog registers as calcite sub-Schemas
of current schema.
+ * The tables in a given database registers as calcite tables
+ * of the [[ExternalCatalogDatabaseSchema]].
+ *
+ * @param catalogIdentifier external catalog name
+ * @param catalog external catalog
+ */
+class ExternalCatalogSchema(
+ catalogIdentifier: String,
+ catalog: ExternalCatalog) extends Schema {
+
+ private val LOG: Logger = LoggerFactory.getLogger(this.getClass)
+
+ /**
+ * Looks up database by the given sub-schema name in the external
catalog,
+ * returns it Wrapped in a [[ExternalCatalogDatabaseSchema]] with the
given database name.
+ *
+ * @param name Sub-schema name
+ * @return Sub-schema with a given name, or null
+ */
+ override def getSubSchema(name: String): Schema = {
+ try {
+ val db = catalog.getDatabase(name)
+ if (db != null) {
+ new ExternalCatalogDatabaseSchema(db.dbName, catalog)
+ } else {
+ null
+ }
+ } catch {
+ case e: DatabaseNotExistException =>
+ LOG.warn(s"database $name does not exist in externalCatalog
$catalogIdentifier")
--- End diff --
**D**atabase
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