A tablesample count executes in two phases:
* Estimate table sizes based on reltuples.
* Based on the estimate:
* If the table is considered 'small', execute an exact relation count.
* Otherwise, count on a sample of the table using TABLESAMPLE.
The size of the sample is chosen in a way that we always roughly scan
the same amount of rows (see TABLESAMPLE_ROW_TARGET).
This commit does a number of things:
1. Reduces the number of queries needed by perform a single query to get all
the tuples for the relevant rows.
2. Uses a transaction to query the tuple counts to ensure that the data
is retrieved from the primary.
Closes#46742
Uses PostgreSQL tuple estimates to provide a much faster yet approximate
count. See https://wiki.postgresql.org/wiki/Slow_Counting for more details.
We only use this fast method if the table has been analyzed or vacuumed
within the last hour.
Closes#46255
Andreas Brandl