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Reinstate HEAP_XMAX_LOCK_ONLY|HEAP_KEYS_UPDATED as allowed

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Commit 866e24d47db1 added an assert that HEAP_XMAX_LOCK_ONLY and
HEAP_KEYS_UPDATED cannot appear together, on the faulty assumption that
the latter necessarily referred to an update and not a tuple lock; but
that's wrong, because SELECT FOR UPDATE can use precisely that
combination, as evidenced by the amcheck test case added here.

Remove the Assert(), and also patch amcheck's verify_heapam.c to not
complain if the combination is found.  Also, out of overabundance of
caution, update (across all branches) README.tuplock to be more explicit
about this.

Author: Julien Rouhaud <rjuju123@gmail.com>
Reviewed-by: Mahendra Singh Thalor <mahi6run@gmail.com>
Reviewed-by: Dilip Kumar <dilipbalaut@gmail.com>
Discussion: https://postgr.es/m/20210124061758.GA11756@nol
This commit is contained in:
Alvaro Herrera 2021-02-23 17:30:21 -03:00
parent 6e6fecf495
commit 2583917075
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1 changed files with 4 additions and 3 deletions
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7
src/backend/access/heap/README.tuplock
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@ -146,9 +146,10 @@ The following infomask bits are applicable:
FOR UPDATE; this is implemented by the HEAP_KEYS_UPDATED bit.
- HEAP_KEYS_UPDATED
This bit lives in t_infomask2. If set, indicates that the XMAX updated
this tuple and changed the key values, or it deleted the tuple.
It's set regardless of whether the XMAX is a TransactionId or a MultiXactId.
This bit lives in t_infomask2. If set, indicates that the operation(s) done
by the XMAX compromise the tuple key, such as a SELECT FOR UPDATE, an UPDATE
that modifies the columns of the key, or a DELETE. It's set regardless of
whether the XMAX is a TransactionId or a MultiXactId.
We currently never set the HEAP_XMAX_COMMITTED when the HEAP_XMAX_IS_MULTI bit
is set.