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https://issues.apache.org/jira/browse/CALCITE-6254?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17818078#comment-17818078
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Barry Kelly commented on CALCITE-6254:
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I've not supported an abbreviated syntax for nullary functions - the
parentheses are always required for invocation in the grammar & parser changes
I'm proposing. This means that functions can still live in a separate namespace
to tables from Calcite's perspective, as they currently can.
Enabling it for nullary table functions (are there many examples of those?)
would mean losing the disjointness of the namespaces, which (a) would be a
breaking change for existing clients, and (b) looks like an expensive change
for the sake of a consistency of dubious utility.
> Enable calling table functions without requiring TABLE() wrapper
> ----------------------------------------------------------------
>
> Key: CALCITE-6254
> URL: https://issues.apache.org/jira/browse/CALCITE-6254
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Affects Versions: 1.36.0
> Reporter: Barry Kelly
> Priority: Minor
> Labels: parser
> Original Estimate: 24h
> Remaining Estimate: 24h
>
> Currently, when selecting from a table function, the function call needs to
> be wrapped in TABLE() like this:
> {{ SELECT * FROM TABLE(table_func('args'));}}
> Many dialects (SQL Server, PostgreSQL, BigQuery) do not require this:
> {{ SELECT * FROM table_func('args');}}
> The current Calcite grammar can be extended to permit this syntax without
> conflicting with other productions.
> There is a close call with the dynamic columns feature used by
> [Phoenix|https://phoenix.apache.org/dynamic_columns.html]
> {{ SELECT * FROM EventLog(lastGCTime TIME)}}
> It can be disambiguated with 3 tokens of lookahead, seeing past the '(' and
> identifier to the presence of a type production. The table extension clause
> for dynamic columns requires at least one field, so the empty case of '()' is
> not ambiguous either.
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