`ALTER TABLE ... FETCH` and `ALTER TABLE ... ATTACH` queries were
disabled in the Replicated database engine, because it could cause
accidental duplication of data.
This enables these queries but without replicating them.
In the case of `FETCH`, the part will only be fetched on the server
where the query is issued.
Similarly, in the case of `ATTACH`, the attached part only needs to
be available on the server where the query is issued.
If the table itself is using one of the Replicated MergeTree engines,
the attached data is then replicated by the table engine itself,
without intervention of the database engine.
This change is meant to help with live backup/restore when using the
Replicated database engine, using FREEZE for backup and ATTACH for
restore.
Add an ability to reset custom setting to default and remove it from table's metadata.
This will allow to rollback the change without knowing the system/config's default.
Signed-off-by: Aleksei Semiglazov <asemiglazov@cloudflare.com>
TODO (suggested by Nikolai)
1. Build query plan fro current query (inside storage::read) up to WithMergableState
2. Check, that plan is simple enough: Aggregating - Expression - Filter - ReadFromStorage (or simplier)
3. Check, that filter is the same as filter in projection, and also expression calculates the same aggregation keys as in projection
4. Return WithMergableState if projection applies
3 will be easier to do with ActionsDAG, cause it sees all functions, and dependencies are direct (but it is possible with ExpressionActions also)
Also need to figure out how prewhere works for projections, and
row_filter_policies.
wip
