imback82 opened a new pull request #26741: [SPARK-30104][SQL] Fix catalog 
resolution for 'global_temp'
URL: https://github.com/apache/spark/pull/26741
 
 
   ### What changes were proposed in this pull request?
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   `global_temp` is used as a database name to access global temp views. The 
current catalog lookup logic considers only the first element of multi-part 
name when it resolves a catalog. This results in using the session catalog even 
`global_temp` is used as a table name under v2 catalog. This PR addresses this 
by making sure multi-part name has two elements before using the session 
catalog.
   
   ### Why are the changes needed?
   <!--
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     1. If you propose a new API, clarify the use case for a new API.
     2. If you fix a bug, you can clarify why it is a bug.
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   Currently, 'global_temp' can be used as a table name in certain commands 
(CREATE) but not in others (DESCRIBE):
   ```
   // Assume "spark.sql.globalTempDatabase" is set to "global_temp".
   sql(s"CREATE TABLE testcat.t (id bigint, data string) USING foo")
   sql(s"CREATE TABLE testcat.global_temp (id bigint, data string) USING foo")
   sql("USE testcat")
   
   sql(s"DESCRIBE TABLE t").show
   +---------------+---------+-------+
   |       col_name|data_type|comment|
   +---------------+---------+-------+
   |             id|   bigint|       |
   |           data|   string|       |
   |               |         |       |
   | # Partitioning|         |       |
   |Not partitioned|         |       |
   +---------------+---------+-------+
   
   sql(s"DESCRIBE TABLE global_temp").show
   org.apache.spark.sql.AnalysisException: Table not found: global_temp;;
     'DescribeTable 'UnresolvedV2Relation [global_temp], 
org.apache.spark.sql.connector.InMemoryTableSessionCatalog@2f1af64f, 
`global_temp`, false
     at 
org.apache.spark.sql.catalyst.analysis.CheckAnalysis.failAnalysis(CheckAnalysis.scala:47)
     at 
org.apache.spark.sql.catalyst.analysis.CheckAnalysis.failAnalysis$(CheckAnalysis.scala:46)
     at 
org.apache.spark.sql.catalyst.analysis.Analyzer.failAnalysis(Analyzer.scala:122)
   ```
   
   ### Does this PR introduce any user-facing change?
   <!--
   If yes, please clarify the previous behavior and the change this PR proposes 
- provide the console output, description and/or an example to show the 
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   Yes, `sql(s"DESCRIBE TABLE global_temp").show` in the above example now 
displays:
   ```
   +---------------+---------+-------+
   |       col_name|data_type|comment|
   +---------------+---------+-------+
   |             id|   bigint|       |
   |           data|   string|       |
   |               |         |       |
   | # Partitioning|         |       |
   |Not partitioned|         |       |
   +---------------+---------+-------+
   ```
   instead of throwing an exception.
   
   ### How was this patch tested?
   <!--
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it was difficult to add.
   -->
   Added new tests.

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