Complete INSERT and UPSERT operations support

What

This issue will track design and implementation of various forms of INSERT operation including UPSERT but excluding bulk inserts and select into new table statements.

Why

Right now we lack support of some insert sub-clauses/variations that are requested by users from time to time. It makes sense perform upfront design and implement it step-by-step.

Initial plans for this feature was to add translation of merge API calls to upsert-based SQL for postgresql and mysql providers but after more detailed review it became clear that it is not possible:

• UPSERT is less flexible compared to Merge API and it is not possible to implement it’s limitations using Merge API
• UPSERT support not limited to MySql and PostgreSQL and supports also databases with native merge support

Related issues

• #869 - generic issue for OUTPUT/RETURNING clause support for various DML operations including INSERT
• #979 - generic issue for RETURNING clause support in PostgreSQL for various DML operations including INSERT
• #1480 - original issue to add UPSERT support though MergeAPI
• #2355 - similar to #1480 : emulation of merge using upsert
• #2528 - INSERT ... ON CONFLICT DO NOTHING feature request

Designs

API Design

Implementation notes and not supported functionality

In general we are not planning to implement following functionality right now:

• insert query options/hints as we have separate task for it (#2603)
• some db-specific features, implemented by single database and not expected to be used a lot. Such stuff could be implemented on request

[viceroypenguin]

Question 1: For SQLite, how is the InsertWith() method different than int Insert<TSource, TTarget>(this IQueryable<TSource> source, ITable<TTarget> target, Expression<Func<TSource, TTarget>> setter) with an IQueryable<> that was created via .AsCte() (other than parameter order)?

[jordan-erisman]

Is there any timeline for this?