Danny Chen commented on CALCITE-3379:

[~julianhyde] [~hyuan],yes, the linked PR is not relevant with this JIRA, maybe 
i mis-clicked.

Let me re-state the use case:

Now user can declare the column of a table in DDL with special strategy(known 
as column constraints in other sql engines[1] [2]), as we enumerate in 
ColumnStrategy[3], then with the SqlNode got from 
InitializerExpressionFactory#newColumnDefaultValue, SqlToRelConverter would 
translate it to a RexCall and put it the project list above the table.

InitializerExpressionFactory#newColumnDefaultValue use the InitializerContext 
to convert the SqlNode, the problem is that:
 # SqlNode got from the DDL is not validated(we have no validation for DDL 
nodes yet, and i think we don'tneed to) and we always got a RexCall with 
 # The SqlNode can not be serialized directly, many times the DDL definition 
need to persist into the Catalog as a metadata(we may unparse it to pure sql 
but that is not intuitive and we still need use to parse it back to SqlNode 
when deserializing)

So, what i proposed is that add the 
InitializerContext#convertExpression(String) and deprecate the 
#convertExpression(SqlNode), Calcite is the role to parse the string to SqlNode 
then validate and translate it to RexCall. What the user see is always a string 
and it can be persisted.


> 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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