Give the pushed down MySQL query a select list

[robinskil]

Problem

A count query on a MySQL external table pushes down a query with an empty select list. MySQL rejects the statement:

ERROR 42000 (1064): You have an error in your SQL syntax; check the manual ... near 'FROM companies WHERE (companies.revenue > 40.0)) AS ...

The plan projects no column. The generated SQL puts FROM directly after SELECT.

Steps

1. Create an external table with STORED AS MYSQL.
2. Run SELECT count(*) FROM <table> WHERE revenue > 40.
3. The server answers 400.

The failure repeats when the file runs alone.

Failed test

• test_mysql.py::test_mysql_filter_result

Task

1. Emit SELECT 1 when the pushed down plan projects no column.
2. Add a test for a count query with a filter.

Environment

Branch features/reimplement-atlas. Image beacon-integration:local.