cloud-fan commented on issue #25955: [SPARK-29277][SQL] Add early DSv2 filter 
and projection pushdown
URL: https://github.com/apache/spark/pull/25955#issuecomment-544617970
 
 
   This is a tricky problem and we need to think through it carefully. The 
table lookup logic is not consistently defined within Spark SQL:
   1. for SELECT/INSERT, we try to lookup temp view first, then lookup 
table/view(these two share the same namespace so doesn't matter which one goes 
first). This is a little weird that Spark can insert into a temp view, but 
that's what it is.
   2. for some commands like DESC TABLE, we follow the previous one and also 
try to lookup temp view first, because these commands support temp view.
   3. for other commands like ALTER TABLE, we try to lookup table/view, because 
they don't support temp view.
   
   Seems like this PR is trying to unify the table lookup logic, which may 
change the behavior. For example, if there are a table and a temp view have the 
same name, ALTER TABLE should still work, but it fails now since both 
`ResolveCatalog` and `ResolveCatalogs` skip temp view.

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