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https://issues.apache.org/jira/browse/CALCITE-3379?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16948301#comment-16948301
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Danny Chen commented on CALCITE-3379:
-------------------------------------
Finally i decide not to support a pure string column expression, if user want
to persist their expressions, they can firstly transform them to sql strings
like "a + 1" or "my_udf(a)", then use the SqlParser to parse it back to SqlNode
and transform it to RexNode with the InitializerContext.
That means, use should control the expressions se-de by themselves. Personally
i think the SqlToRelConverter should not see the parser thing anymore.
So i would just close this issue.
> 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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