[ENH] substr() not supported in dask-sql

Is your feature request related to a problem? Please describe. I’m working on porting a large set of queries from another engine to dask-sql. I see that I can update the queries to use “substring” instead, but it would be nice if users didn’t have to.

Describe the solution you’d like Can we have substr() supported in dask-sql in the same way that substring() is?

Describe alternatives you’ve considered substring() works in dask-sql not substr(). However, we do not want to alter the sql files by changing substr() to substring()

Additional context Here’s an example query I’d like to be able to run: `import cudf from dask_sql import Context

  dc = Context()
  
  df = cudf.DataFrame({'s_c': ['ATX', 'LAX', 'SFO'], 's_d':['38714','37206','38714'], 
                       'd_d':['1900-01-01','1900-01-04','2199-12-28']})
  dc.create_table('my_table', df) 
  
 
  query = """
            select substr(s_c,1,30)
            from
             (select s_c
              from my_table
              where s_d = d_d           
              group by s_c)
          """
  
  print(dc.sql(query).compute())`

[ayushdg]

Thanks for sharing. This confirms @beckernick comment on adding support for the syntax from the java side.

@DaceT On an unrelated note regarding GitHub formatting I notice that you’re using the single backtick (`) for code formatting. If you want to format multiline outputs like the one above, you can try using three backticks (```) instead of one for the formatting around the codeblock. I typically refer to these docs when I can’t recall some of the GitHub formatting rules.

[beckernick]

SUBSTR is not part of the default SQL standard. It may need to be enabled on the SqlLibrary enum which is used to create the operator table, though it looks like it should be here via Postgres.

• https://github.com/dask-contrib/dask-sql/blob/7b60e4a732864f1a9b85f5d52524badd643fd46a/planner/src/main/java/com/dask/sql/application/RelationalAlgebraGenerator.java#L173-L180
• https://github.com/apache/calcite/blob/4bc916619fd286b2c0cc4d5c653c96a68801d74e/core/src/main/java/org/apache/calcite/sql/fun/SqlLibrary.java#L45-L65
• https://calcite.apache.org/docs/reference.html#dialect-specific-operators