[tarantool-patches] Re: [PATCH 1/2] sql: fix collation node duplication in AST

[Vladislav Shpilevoy <v.shpilevoy@tarantool.org>]

Hi! Thanks for the patch! Looks like the problem in the patch
is much easier to reproduce, without 'EXCEPT':

    box.execute('SELECT s COLLATE "unicode_ci" FROM a ORDER BY 1 COLLATE "unicode_ci"')

Just use ORDER BY <number> + COLLATE.

But having got this simpler test I found that you broke another one.
Look at this schema:

    box.cfg{}
    box.execute('CREATE TABLE a (id INT PRIMARY KEY, s TEXT)')
    box.execute("INSERT INTO a VALUES (1, 'B'), (2, 'b')")

Before your patch:

    tarantool> box.execute('SELECT s COLLATE "unicode_ci" FROM a ORDER BY 1 COLLATE "unicode"')
    ---
    - metadata:
      - name: s COLLATE "unicode_ci"
        type: string
      rows:
      - ['b']
      - ['B']
    ...


After your patch:

    tarantool> box.execute('SELECT s COLLATE "unicode_ci" FROM a ORDER BY 1 COLLATE "unicode"')
    ---
    - metadata:
      - name: s COLLATE "unicode_ci"
        type: string
      rows:
      - ['B']
      - ['b']
    ...

Result set order has changed. This is because you ignore a new collation
regardless of a name of a previous one. It means, that the patch should not
be applied as is.

We should either

    1) Replace the old collation with a new one, as an optimization;
    2) Left as is now.

I vote for the second as the simplest, and add a test provided by me
above to ensure we will never break it accidentally.

This commit should consist of the test and a comment in resolveAlias.
And keep this nice ASCII schema.