Something odd came up during this review, which Martin Kleppmman previously reported in https://github.com/ept/hermitage/blob/master/postgres.md: PostgreSQL repeatable read isn't repeatable read: it's snapshot isolation, which allows a specific class of G2-item anomalies (those with nonadjacent rw edges) prohibited under formalizations of repeatable read.
This could still be consistent with the ANSI SQL standard's definition of repeatable read! As Berenson et al. pointed out twenty five years ago (!), the standard is ambiguous. The paper argues that there are two interpretations of the ANSI phenomena: strict, and broad. They say the broad interpretation is what ANSI *meant* to define, and goes on to define and analyze snapshot isolation in those terms. https://www.microsoft.com/en-us/research/wp-content/uploads/2016/02/tr-95-51.pdf
So, while Berenson et al. say that snapshot isolation isn't stronger than repeatable read, PostgreSQL appears to have implicitly adopted the strict interpretation instead, and says that SI is stronger than RR. In fact, SI prohibits *every* anomaly in the strict interpretation of the ANSI SQL standard, including their (narrow) definition of phantoms!
Anyway, consistency models are a mess; news at 11. 😂
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