### What changes were proposed in this pull request?
`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?
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.InMemoryTableSessionCatalog2f1af64f, `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?
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?
Added new tests.
Closes#26741 from imback82/global_temp.
Authored-by: Terry Kim <yuminkim@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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