Github user fhueske commented on a diff in the pull request:
https://github.com/apache/flink/pull/3406#discussion_r104371681
--- 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")
+ null
+ }
+ }
+
+ /**
+ * Lists the databases of the external catalog,
+ * returns the lists as the names of this schema's sub-schemas.
+ *
+ * @return names of this schema's child schemas
+ */
+ override def getSubSchemaNames: util.Set[String] =
catalog.listDatabases().toSet.asJava
+
+ override def getTable(name: String): Table = null
+
+ override def isMutable: Boolean = true
+
+ override def getFunctions(name: String): util.Collection[Function] =
+ util.Collections.emptyList[Function]
+
+ override def getExpression(parentSchema: SchemaPlus, name: String):
Expression =
+ Schemas.subSchemaExpression(parentSchema, name, getClass)
+
+ override def getFunctionNames: util.Set[String] =
util.Collections.emptySet[String]
+
+ override def getTableNames: util.Set[String] =
util.Collections.emptySet[String]
+
+ override def contentsHaveChangedSince(lastCheck: Long, now: Long):
Boolean = false
--- End diff --
Right, my understanding of the parameter is that Calcite asks "Has the
content changed?" and if we say "false" it will look into the cache. If we tell
Calcite "true" (the content has changed) it will go against the external
catalog. That's why I was thinking that `true` is the right return value. Can
you check Calcite's behavior for this function?
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