UPSERT with single database statement

[AlekSi]

Several RDBMS support UPSERT (INSERT or UPDATE with a single statement):

• PostgreSQL 9.5+: docs, wiki.
• MySQL 5.5+: docs.
• SQLite3: docs.

We should add Upsert method to reform. It should use a single RDBMS-specific statement. Behaviour could be RDBMS-specific too but well documented (see PostgreSQL wiki link for the good overview). It should not try to emulate it in unsupported RDBMS with several queries and/or transactions, it will return an error instead.

WIP branch that could be useful: https://github.com/go-reform/reform/tree/upsert.

[AlekSi]

@AlphaB What is your usecase?

[AlphaB]

@AlekSi I have a table with id (pk), user_id (reference to users table), entity_id and some other columns. There is a unique index on user_id and entity_id columns and I want to be able to upsert records into this table by user_id and entity_id instead of making SELECT and INSERT/UPDATE. That's it.