Rohan Garg created CALCITE-5074:
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Summary: Allowing parser extensions
Key: CALCITE-5074
URL: https://issues.apache.org/jira/browse/CALCITE-5074
Project: Calcite
Issue Type: New Feature
Reporter: Rohan Garg
We (I and [~cheddar]) recently had the need to extend various parts of Calcite
in a non-standard SQL manner in our project which uses Calcite. For example,
Calcite's TABLESAMPLE keyword always takes a percentage to sample and we wanted
to adjust it to also allow for an explicit number of rows. Given that this is
non-standard, we felt that it wouldn't make sense to do a PR that actually
impacts Calcite's normal behavior, so we sought a method of extending Calcite
to add support for our capabilities while still benefiting from all that
Calcite offers.
We came up with an idea for having an "override map" that can be provided in
configuration and will cause the parser to override a specific production rule.
We include a commit link to show the idea in action
(https://github.com/rohangarg/calcite/commit/7f0c6ad8c8f6bb2a1d1cca025d35670a8c62b3c4).
It's a bit fiddly in that each override point needs to be extended with
if/else template syntax, but that's a pattern that already seems to exist and
maybe that's a feature rather than a bug? In either case, if that's too
fiddly, but this general pattern makes sense, a subsequent task could be taken
on to try to see if there's another point that this could be added more
generically without the need for adding the if/else in the template. Does this
seem like something that could be merged?
We have included an override grammar that also relaxes the percentage
constraints on TABLESAMPLE as an example of the type of customization that this
is attempting to enable.
Just for reference, this customization does exist in other systems as well, so
it's non-standard but also not unheard of:
1. MS SQL Server :
https://docs.microsoft.com/en-us/sql/t-sql/queries/from-transact-sql?view=sql-server-ver15#tablesample-clause
2. Snowflake :
https://docs.snowflake.com/en/sql-reference/constructs/sample.html
3. Google Spanner :
https://cloud.google.com/spanner/docs/reference/standard-sql/query-syntax#tablesample_operator
4. Apache Spark :
https://spark.apache.org/docs/latest/sql-ref-syntax-qry-select-sampling.html
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