Question 1391

One Cell Is Wrong

A team wiki carries this table, labelled "isolation levels in our PostgreSQL databases". Exactly one cell is wrong for PostgreSQL specifically, as opposed to for the SQL standard. Which one?

Level Dirty read Non-repeatable read Phantom read Serialization anomaly
Read uncommitted not possible possible possible possible
Read committed not possible possible possible possible
Repeatable read not possible not possible possible possible
Serializable not possible not possible not possible not possible
A

Serializable, serialization anomaly. A set of Serializable transactions can still commit an anomaly when none of them writes.

B

Repeatable read, phantom read. The standard permits phantoms at that level, but PostgreSQL's Repeatable Read is snapshot isolation and does not allow them.

C

Read uncommitted, dirty read. PostgreSQL does permit dirty reads at Read Uncommitted; the row has it backwards.

D

Repeatable read, serialization anomaly. Once non-repeatable reads and phantoms are ruled out there is no anomaly left for that level to allow.