Implement support for TABLESAMPLE

[Nick Pope]

PostgreSQL 9.5 added support for the TABLESAMPLE feature from the SQL:2003 standard. It would be useful to add support for this to Django.

For PostgreSQL it would also be good to support the custom tsm_system_rows and tsm_system_time extensions to provide SYSTEM_ROWS and SYSTEM_TIME in addition to BERNOULLI and SYSTEM defined in the standard.

Note that TABLESAMPLE can also be used to update or delete only a sample of rows.

I suggest adding .tablesample(method, value[, seed]) to QuerySet where:

• method is one of bernoulli, system, rows or time,
• value is the percentage, count or time argument to pass,
• seed is the value to use for the REPEATABLE option.

Note that seed should not be allowed with rows or time methods -- see links to extension documentation below.

Minimum versions for supported databases:

• PostgreSQL 9.5
• Microsoft SQL Server 2005 -- only supports system (percent) and rows (count)

References:

• ​http://www.neilconway.org/talks/hacking/ottawa/sql_standard.pdf
• ​https://wiki.postgresql.org/wiki/TABLESAMPLE_Implementation
• ​http://blog.2ndquadrant.com/tablesample-in-postgresql-9-5-2/
• ​http://www.depesz.com/2015/05/23/waiting-for-9-5-tablesample-sql-standard-and-extensible/
• ​http://www.postgresql.org/docs/9.5/static/tsm-system-time.html
• ​http://www.postgresql.org/docs/9.5/static/tsm-system-rows.html
• ​https://msdn.microsoft.com/en-us/library/ms189108(v=sql.105).aspx

[Tim Graham]

Before doing any coding, please create a thread on the DevelopersMailingList to get feedback on the proposed APIs and add a link to it in this ticket.