Allow defining order of column for indexes

[anonymous]

Hi,

Currently the index_together tuples do not support index ordering using + / -.

If I have the following table:

class Messages:
   user_id,
   rating,
   last_update,

   index_together=[
     ['user_id', 'rating',  'last_update',] # sorted ASC by default
  ]

So for MySQL, the following query cannot make use of the index:

Messages.objects.filter(user_id=xxx).order_by('+rating','-last_update')

Pardon me if it actually can make use of the index.

I know there are workarounds, such as negating values to make a DESC ordering become ASC ordering.
But it is just nice to have such a feature.
Btw, what is the reason for not having this feature?

[wim@…]

Hi anonymous, thanks for taking the time to fill in a ticket. Just to answer your question: most features are not implemented because people did not have time to write the code for it. :) So if you are willing to write a pull request and contribute such a feature could land in Django.

Unfortunately I do not understand MySQl enough to mark this ticket as accepted.

[Anssi Kääriäinen]

To me this seems acceptable. Of course, supporting this only on MySQL isn't good, I believe most backends support index ordering.

[Akshesh Doshi]

PR: ​https://github.com/django/django/pull/6982

Although requested for alter_index_together, I have implemented the feature with the new class based indexes which would allow users to do the same thing.

The patch contains implementation for all databases except MySQL which doesn't support the feature itself according to their ​official docs. I felt this to be a bit weird as originally the feature had been requested specifically for MySQL. Please let me know if I missed something in this context.

The patch doesn't add the feature for unique_together as I couldn't understand/don't know what that means. I can try to add it for unique_together as well if somebody could explain that.

[Tim Graham]

I'll retitle the ticket and we can open a new one if there's more to be done with respect to unique_together.

[Tim Graham <timograham@…>]

In f842d10:

Refs #20888 -- Added index order introspection.

[Tim Graham <timograham@…>]

In 311a8e8:

Fixed #20888 -- Added support for column order in class-based indexes.

[Akshesh Doshi]

Since technically we haven't fixed the ticket described by the earlier summary, but in the end it's the feature that matters so won't reopen this and renaming it instead.

[Tim Graham]

We need to revisit this as on PostgreSQL 9.6, "column am.amcanorder does not exist".

According to a post on the PostgreSQL mailing list: "Using pg_get_indexdef() would leave you much less vulnerable to cross- version differences in the system catalogs.

[Tim Graham]

I drafted a patch based on ​a commit for pgjdbc but it's not entirely working yet and has issues on some older versions of PostgreSQL. I didn't spend too much time debugging.

[François Freitag]

​PR

PostgreSQL 9.6 gets rid of most of pg_am columns[0]. In particular, amcanorder disappears and should be replaced with calls to pg_index_column_has_property(). PostgreSQL versions < 9.6 do not provide this method, hence we are forced to rely on a brittle query.

Since only B-tree indexes support ordering[1], this is the only type of index to consider. This solution relies on the access method implementation[2] to determine whether ASC or DESC ordering is in use.

Let's hope that a new view is added to information_schema[3] in the future.

[0] ​https://www.postgresql.org/docs/9.6/static/release-9-6.html
[1] ​https://www.postgresql.org/docs/current/static/indexes-ordering.html
[2] ​https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/include/catalog/pg_index.h;h=ee97c5dec836ad82c5b75440fcba51d4c106fc33;hb=HEAD#l99
[3] ​http://stackoverflow.com/questions/18121103/how-to-get-the-index-column-orderasc-desc-nulls-first-from-postgresql#comment64529591_18128104

[Tim Graham <timograham@…>]

In 86bb9a05:

Refs #20888 -- Fixed index ordering introspection on PostgreSQL 9.6.