imback82 opened a new pull request #26921: [SPARK-30282][SQL] 
UnresolvedV2Relation should be resolved to temp view first
URL: https://github.com/apache/spark/pull/26921
 
 
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   ### What changes were proposed in this pull request?
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   This is a part of effort to make the relation lookup behavior consistent: 
[SPARK-2990](https://issues.apache.org/jira/browse/SPARK-29900).
   
   This PR specifically addresses the V2 commands whose logical plan contains 
`UnresolvedV2Relation` such that if `UnresolvedV2Relation` is resolved to a 
temp view, those commands should error out with a message that v2 command 
cannot handle temp views.
   ### Why are the changes needed?
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     1. If you propose a new API, clarify the use case for a new API.
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   For the following v2 commands, `Analyzer.ResolveTables` does not check 
against the temp views before resolving `UnresolvedV2Relation`, thus it always 
resolves `UnresolvedV2Relation` to a table:
   ```
   ALTER TABLE
   DESCRIBE TABLE
   SHOW TBLPROPERTIES
   ```
   Thus, in the following example, `t` will be resolved to a table, not a temp 
view:
   ```
   sql("CREATE TEMPORARY VIEW t AS SELECT 2 AS i")
   sql("CREATE TABLE testcat.ns.t USING csv AS SELECT 1 AS i")
   sql("USE testcat.ns")
   sql("DESCRIBE t") // 't' is resolved to a table
   ```
   This behavior is inconsistent with other commands which look up temp views 
first.
   
   ### Does this PR introduce any user-facing change?
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   If yes, please clarify the previous behavior and the change this PR proposes 
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   Yes, now the above example will fail as follows:
   ```
   sql("DESCRIBE t") // 't' is now resolved to a temp view
   org.apache.spark.sql.AnalysisException: A temp view 't' cannot be handled by 
V2 commands.;
     at 
org.apache.spark.sql.catalyst.analysis.Analyzer$ResolveTables$.org$apache$spark$sql$catalyst$analysis$Analyzer$ResolveTables$$resolveV2Relation(Analyzer.scala:782)
   ```
   
   ### How was this patch tested?
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   Added new tests.

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