Fixing error on deletes from owning table for a lookup being populated by CreateLookupVindex after a SwitchWrites

[frouioui]

Description

After creating two keyspaces and a lookup Vindex using CreateLookupVindex , every delete query that uses the same lookup Vindex starts failing. Below are: a sample query using the column c2 (which is our lookup Vindex); and the error that we got after executing the query.

mysql> delete from table where c2 = 1;
ERROR 1105 (HY000): cannot map vindex to unique keyspace id: DestinationKeyRange(-)

The reason behind this error is the following: the engine tries to execute a delete equal however, the lookup Vindex used to resolve the shard is in write_only mode, meaning we cannot resolve to which shard this query needs to be routed. The default behavior, when this happened, was to return a slice of DestinationKeyRange keyspace IDs, which is not a single shard, thus failing for delete equal. This pull request changes this behavior by transforming delete unique into delete scatter when the query is made using a lookup Vindex on write_only mode.

Related Issue(s)

Checklist

• Should this PR be backported?
• Tests were added or are not required
• Documentation was added or is not required

[aquarapid]

LGTM; also validated manually with some edgecases.

[deepthi]

Does this also fix #7336?

[harshit-gangal]

Does this also fix #7336?

It does not fix the issue mentioned.

[frouioui]

@harshit-gangal, as we talked about offline: I have modified the code to change the DML's opcode in the planbuilder instead of doing it in the engine

[harshit-gangal]

we can remove the changes in the engine test as the changes are associated with the planner.
Also, add unit test at executor level.

[frouioui]

@harshit-gangal, thank you for the review! I have applied the recommended changes

[harshit-gangal]

changes look good.
More tests are needed in executor and planner where a higher cost Vindex is selected as the lower cost vindex was in backfill.