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https://issues.apache.org/jira/browse/CALCITE-3379?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16948295#comment-16948295
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Danny Chen commented on CALCITE-3379:
-------------------------------------

Okey, got your idea, personally, i'm inclined to validate the nodes when the 
query really happens, just like many other sql engines do to VIEWs [1]. The 
computed column actually is a VIEW.

[1] 
https://docs.microsoft.com/en-us/sql/t-sql/statements/create-view-transact-sql?view=sql-server-2017#remarks

> Support expand STRING column expression in table during sql-to-rel conversion
> -----------------------------------------------------------------------------
>
>                 Key: CALCITE-3379
>                 URL: https://issues.apache.org/jira/browse/CALCITE-3379
>             Project: Calcite
>          Issue Type: Improvement
>          Components: core
>    Affects Versions: 1.21.0
>            Reporter: Danny Chen
>            Assignee: Danny Chen
>            Priority: Major
>              Labels: pull-request-available
>             Fix For: 1.22.0
>
>          Time Spent: 2h
>  Remaining Estimate: 0h
>
> Now there are 2 ways to convert a RelOptTable to LogicalTableScan:
> 1. One way is to open the Config#isConvertTableAccess[1] flag and the 
> SqlToRelConverter would invoke the #toRel method which transforms the table 
> to a node returned by the user(Usually a table scan).
> 2. Another way is to use the LogicalTableScan rule, this rule would invoke 
> RelOptTable#toRel and wrap the returned node with a LogicalTableScan.
>  
> The difference between 1 and 2 is that, 2 happens in the planning rule but 1 
> happens in sql-to-rel conversion, 1 also supports to expand the table columns 
> based on the defined default values expressions, see 
> InitializerExpressionFactory#newColumnDefaultValue.
>  
> The problem with the InitializerExpressionFactory#newColumnDefaultValue is 
> that it uses InitializerContext#convertExpression to convert a SqlNode, but 
> if the SqlNode is not validated, we always got a RexCall with 
> SqlUnresolvedFunction. We should give the user chance to validate their 
> SqlNode or even we can support pure string expressions which can be used to 
> persist.
>  
> Another problem with #toRel is that after the expressions applied as a 
> projection, user has no chance to apply any other relational nodes if they 
> want, we can actually support this, the same way as we support the column 
> expressions.
>  
> [1]https://github.com/apache/calcite/blob/2dc97e6723e1b5bf762540f87ffffb5cd1a848a1/core/src/main/java/org/apache/calcite/sql2rel/SqlToRelConverter.java#L5605



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