Add tag support for sql

[m0sth8]

Usually, it's very useful to mark queries with some tags.

This code db.Tag("disable_user:%s", userId).Update(user) might produce the next query UPDATE /* disable_user:10 */ ...

[AlekSi]

Do you have any examples in existing ORMs?

[AlekSi]

@m0sth8 shared some details privately. The idea is to generate SQL statements with embedded comments, so it is possible to track them from RDMS logs back to application code. Sounds like a useful feature.

How RDMS support comments:

• PostgreSQL supports /* */ sytax without problems. Optimizer hints are not supported.
• MySQL supports /* */ syntax. It has hacks for MySQL specific code (/*! STRAIGHT_JOIN */) with optional version check (/*!50110 KEY_BLOCK_SIZE=1024 */) and optimizer hints (/*+ BKA(t1) */) after keywords SELECT, UPDATE, etc. Internets say that comments in statements can kill query cache: 1, 2, 3.
• SQLite3 supports /* */ syntax. Not sure about how it affects cache, but it should not be a concern for SQLite3 user.
• Microsoft SQL Server supports /* */ syntax. Comments can kill query cache: 1, 2.
• Oracle supports /* */ syntax with hacks for optimizer hints (/*+ ALL_ROWS */) after keywords SELECT, UPDATE, etc.

How it should be implemented is an open question. Querier looks like a natural place, but then statements like BEGIN from DB will not be logged.