Support for get_schema_names in Dialect

[AchilleAsh]

ClickHouseDialect.get_schema_names was not implemented: its role is just to list the databases.

The implementation already exists in cloudfare sqlalchemy-clickhouse driver so it does make sense to also support it here.

[xzkostyan]

What's the difference between schema and database then?

It's better not to show schemas at all. There will be less misunderstanding in terminology.

[AchilleAsh]

Hi,

Let me give you 2 examples :

MySQL

On MySQL, database is equivalent to schema: one database can only have one schema,
so SHOW schemas is the same as SHOW databases

Details on the documentation: https://mariadb.com/kb/en/library/show-databases/

The SQLA Dialect is therefore straightforward for MySQL:

@reflection.cache  
def get_schema_names(self, connection, **kw):  
    rp = connection.execute("SHOW schemas")  
    return [r[0] for r in rp]

Postgresql

On Postgresql, one database can support multi schemas, then each schema has table: by default the schema is called public and can be omitted in the query. The schema can be see as a namespace/folder to organize tables.
The SQLA Dialect is implemented using the pg_namespace catalog located in the pg_catalog schema.

You can read more details on the official doc: https://www.postgresql.org/docs/current/static/ddl-schemas.html

Now in our case Clickhouse is not multi schemas so it behave like MySQL: one database has one schema.
Clickhouse has no schema awareness, the only namespace-like function we can achieve is by using databases (just like MySQL): That's why as in MySQL Dialect, the Clickhouse Dialect should return the databases as schemas.

SQLA goal is to provide an abstraction layer on top of multiple SQL DB, some with multi schemas, some without: they had to have the notion of schemas, but here we have to take the shortcut schema == database because that's the Clickhouse logic.

I hope my explications were clear,
Timothé

[xzkostyan]

Thanks!

[coveralls]

Coverage increased (+14.1%) to 96.364% when pulling e4a5b2b on AchilleAsh:master into c35d332 on xzkostyan:master.

[coveralls]

Coverage increased (+14.1%) to 96.364% when pulling e4a5b2b on AchilleAsh:master into c35d332 on xzkostyan:master.