View Unary Logical Operator

View is a logical operator with a single child logical operator.

View is created exclusively when SessionCatalog is requested to find a relation in the catalogs (e.g. when DescribeTableCommand logical command is executed and the table type is VIEW).

// Let's create a view first
// Using SQL directly to manage views is so much nicer
val name = "demo_view"
sql(s"CREATE OR REPLACE VIEW $name COMMENT 'demo view' AS VALUES 1,2")
assert(spark.catalog.tableExists(name))

val q = sql(s"DESC EXTENDED $name")

val allRowsIncluded = 100
scala> q.show(numRows = allRowsIncluded)
+--------------------+--------------------+-------+
|            col_name|           data_type|comment|
+--------------------+--------------------+-------+
|                col1|                 int|   null|
|                    |                    |       |
|# Detailed Table ...|                    |       |
|            Database|             default|       |
|               Table|           demo_view|       |
|               Owner|               jacek|       |
|        Created Time|Thu Aug 30 08:55:...|       |
|         Last Access|Thu Jan 01 01:00:...|       |
|          Created By|         Spark 2.3.1|       |
|                Type|                VIEW|       |
|             Comment|           demo view|       |
|           View Text|          VALUES 1,2|       |
|View Default Data...|             default|       |
|View Query Output...|              [col1]|       |
|    Table Properties|[transient_lastDd...|       |
|       Serde Library|org.apache.hadoop...|       |
|         InputFormat|org.apache.hadoop...|       |
|        OutputFormat|org.apache.hadoop...|       |
|  Storage Properties|[serialization.fo...|       |
+--------------------+--------------------+-------+

View is a MultiInstanceRelation so a new instance will be created to appear multiple times in a physical query plan. When requested for a new instance, View creates new instances of the output attributes.

View is considered resolved only when the child is.

View has the following simple description (with state prefix):

View ([identifier], [output])
val name = "demo_view"
sql(s"CREATE OR REPLACE VIEW $name COMMENT 'demo view' AS VALUES 1,2")
assert(spark.catalog.tableExists(name))

val q = spark.table(name)
val qe = q.queryExecution

val logicalPlan = qe.logical
scala> println(logicalPlan.simpleString)
'UnresolvedRelation `demo_view`

val analyzedPlan = qe.analyzed
scala> println(analyzedPlan.numberedTreeString)
00 SubqueryAlias demo_view
01 +- View (`default`.`demo_view`, [col1#33])
02    +- Project [cast(col1#34 as int) AS col1#33]
03       +- LocalRelation [col1#34]

// Skip SubqueryAlias
scala> println(analyzedPlan.children.head.simpleString)
View (`default`.`demo_view`, [col1#33])
Note
 View is resolved by ResolveRelations logical resolution.
Note
 AliasViewChild logical analysis rule makes sure that the output of a View matches the output of the child logical operator.
Note
 EliminateView logical optimization removes (eliminates) View operators from a logical query plan.
Note
 Inserting into a view is not allowed.

Creating View Instance

View takes the following when created:

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