When calling the `cagg_watermark` function to get the watermark of a
Continuous Aggregate we execute a `SELECT MAX(time_dimension)` query
in the underlying materialization hypertable.
The problem is that a `SELECT MAX(time_dimention)` query can be
expensive because it will scan all hypertable chunks increasing the
planning time for a Realtime Continuous Aggregates.
Improved it by creating a new catalog table to serve as a cache table
to store the current Continous Aggregate watermark in the following
situations:
- Create CAgg: store the minimum value of hypertable time dimension
data type;
- Refresh CAgg: store the last value of the time dimension materialized
in the underlying materialization hypertable (or the minimum value of
materialization hypertable time dimension data type if there's no
data materialized);
- Drop CAgg Chunks: the same as refresh cagg.
Closes#4699, #5307
38fcd1b76b