When re-writing query tree, postgres combines multiple subscripting
operators referencing to the same attribute into a single `TargetEntry`
by linking `SubscriptingRef` objects to each other via their `refexpr`
fields. (See `rewriteTargetListIU` & `process_matched_tle` functions.)
However, ruleutils function `get_update_query_targetlist_def` doesn't
know how to properly deparse such a `TargetEntry`. As a result, we were
only taking the last set-by-subscript operation into account when
generating the shard query for such an `UPDATE` command.
In postgres, this doesn't cause any problems (e.g.: when generating
definition of a rule based object) since the query-rewrite
transformations aren't performed on the query tree that
`get_update_query_targetlist_def` is expected to process.
For this reason; with this commit, before processing the target entry
list in our ruleutils based deparser, we first expand such target
entries into multiple ones.
To detect such `SubscriptingRef` objects, we also need to investigate
`FieldStore` and `CoerceToDomain` objects as postgres functions
processing `SubscriptingRef` objects do --although they do so for
different purposes. However, given that Citus already doesn't allow
`INSERT/UPDATE` via `FieldStore`, we only do that for `CoerceToDomain`
objects.
Onur Tirtir