Allow READ COMMITTED isolation level

[rafiss]

CockroachDB is adding support for READ COMMITTED, so it should be allowed here.

See https://www.cockroachlabs.com/docs/stable/read-committed

[gordthompson]

Hi @rafiss - Can you test show transaction isolation level; on v23.2.0? (I'm stuck on v23.2.0-rc.2 until next week.) It doesn't seem to reflect the change from SERIALIZABLE to READ COMMITTED.

gord@gord-dv7:~$ ./crdb_sql.sh
#
# Welcome to the CockroachDB SQL shell.
# All statements must be terminated by a semicolon.
# To exit, type: \q.
#
# Server version: CockroachDB CCL v23.2.0-rc.2 (x86_64-pc-linux-gnu, built 2024/01/08 19:58:04, go1.21.5 X:nocoverageredesign) (same version as client)
# Cluster ID: 8d37ab91-5df7-4ef0-853e-726add540b27
#
# Enter \? for a brief introduction.
#
root@localhost:26257/defaultdb> begin;
BEGIN

Time: 1ms total (execution 1ms / network 1ms)

root@localhost:26257/defaultdb  OPEN> show transaction isolation level;
  transaction_isolation
-------------------------
  serializable
(1 row)

Time: 1ms total (execution 1ms / network 0ms)

root@localhost:26257/defaultdb  OPEN> SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
SET TRANSACTION

Time: 1ms total (execution 1ms / network 0ms)

root@localhost:26257/defaultdb  OPEN> show transaction isolation level;
  transaction_isolation
-------------------------
  serializable
(1 row)

Time: 1ms total (execution 1ms / network 0ms)

[rafiss]

@gordthompson in v23.2, READ COMMITTED is still a preview feature, so there's one more setting to enable before your example will work. try: SET CLUSTER SETTING sql.txn.read_committed_isolation.enabled = true;

[gordthompson]

lgtm