SQL Standard TABLESAMPLE clause

The syntax is

<table factor> ::= <table primary> [ <sample clause> ]

<sample clause> ::= TABLESAMPLE <sample method> <left paren> <sample percentage> <right paren>

<sample method> ::= BERNOULLI | SYSTEM

<sample percentage> ::= <numeric value expression>

The goal is

• support BERNOULLI
  • internally it'll just be a full rnd_next() table scan
• support SYSTEM
  • this will either fall back to Bernoulli if the percentage is too high
  • or use a new interface, say, handler::ha_get_random_row() which will use a random dive into a b-tre, which in MyISAM/Aria will use a "reverse records_in_range" approach. Which is skewed, but so is records_in_range(). Fixing both is beyond the scope of this task.

Implementation order

1. syntax, only with SYSTEM
2. handler interface and storage engine implementation for random dives
3. Bernoulli