PostgreSQL merge query executes as a single native merge query

PostgreSQL merge queries now execute as a single native MERGE statement

On PostgreSQL, $connection->merge($table)->...->execute() now runs one native MERGE statement. Before this change, the driver used the generic emulation: a SELECT followed by an INSERT or an UPDATE. The native statement uses RETURNING merge_action() to report the action taken.

Return values are unchanged: Merge::STATUS_INSERT, Merge::STATUS_UPDATE, or NULL when the row existed and no update was requested. Modules need no code changes.

The generic emulation is still used when:

• the condition targets a table other than the merge table;
• there are no fields to insert;
• values are inserted into serial fields.

When a concurrent transaction inserts the same row during the merge, the statement is retried once. The racing row is then matched and updated.

Casting a pgsql merge query to a string now returns the MERGE SQL when the native path applies. It throws for the emulated path, as before.

Other database drivers are unaffected.