UnresolvedCatalogRelation Leaf Logical Operator — Placeholder of Catalog Tables

UnresolvedCatalogRelation is a leaf logical operator that acts as a placeholder in a logical query plan until FindDataSourceTable logical evaluation rule resolves it to a concrete relation logical plan (i.e. a LogicalRelation for a data source table or a HiveTableRelation for hive table).

UnresolvedCatalogRelation is created when SessionCatalog is requested to find a relation (for DescribeTableCommand logical command or ResolveRelations logical evaluation rule).

// non-temporary global or local view
// database defined
// externalCatalog.getTable returns non-VIEW table
// Make the example reproducible
val tableName = "t1"
  db = "default",
  table = tableName,
  ignoreIfNotExists = true,
  purge = true)

scala> :type spark.sessionState.catalog

import org.apache.spark.sql.catalyst.TableIdentifier
val plan = spark.sessionState.catalog.lookupRelation(TableIdentifier(tableName))
scala> println(plan.numberedTreeString)
00 'SubqueryAlias t1
01 +- 'UnresolvedCatalogRelation `default`.`t1`, org.apache.hadoop.hive.ql.io.parquet.serde.ParquetHiveSerDe

When created, UnresolvedCatalogRelation asserts that the database is specified.

UnresolvedCatalogRelation can never be resolved and is converted to a LogicalRelation for a data source table or a HiveTableRelation for hive table at analysis phase.

UnresolvedCatalogRelation uses an empty output schema.

UnresolvedCatalogRelation takes a single CatalogTable when created.

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