Add index row estimation is not fully usable

[tangenta]

Enhancement

According to the add-index test result, wider tables are more likely to encounter bottlenecks at the table reader, and CPU utilization could be very low. #52000 adopts a strategy to determine reader concurrency: it used a statistic cache to estimate average row size of a table, and adjust reader count based on it.

However, statistic cache is not always available. For example, if a table has never executed ANALYZE, there is no data in statistic cache. For large tables, even if we analyze them with a low sample rate, it is still time-consuming.

We should investigate another way to estimate average row size efficiently.