Commits on Mar 22, 2023

1. Improve Realtime Continuous Aggregate performance …

   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 timescale#4699, timescale#5307

   

   fabriziomello committed Mar 22, 2023
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