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Commit28f4b61

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Reinstate HEAP_XMAX_LOCK_ONLY|HEAP_KEYS_UPDATED as allowed
Commit 866e24d47db1 added an assert that HEAP_XMAX_LOCK_ONLY andHEAP_KEYS_UPDATED cannot appear together, on the faulty assumption thatthe latter necessarily referred to an update and not a tuple lock; butthat's wrong, because SELECT FOR UPDATE can use precisely thatcombination, as evidenced by the amcheck test case added here.Remove the Assert(), and also patch amcheck's verify_heapam.c to notcomplain if the combination is found. Also, out of overabundance ofcaution, update (across all branches) README.tuplock to be more explicitabout 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
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‎src/backend/access/heap/README.tuplock‎

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@@ -146,9 +146,10 @@ The following infomask bits are applicable:
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FOR UPDATE; this is implemented by the HEAP_KEYS_UPDATED bit.
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- HEAP_KEYS_UPDATED
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This bit lives in t_infomask2. If set, indicates that the XMAX updated
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this tuple and changed the key values, or it deleted the tuple.
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It's set regardless of whether the XMAX is a TransactionId or a MultiXactId.
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This bit lives in t_infomask2. If set, indicates that the operation(s) done
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by the XMAX compromise the tuple key, such as a SELECT FOR UPDATE, an UPDATE
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that modifies the columns of the key, or a DELETE. It's set regardless of
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whether the XMAX is a TransactionId or a MultiXactId.
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We currently never set the HEAP_XMAX_COMMITTED when the HEAP_XMAX_IS_MULTI bit
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is set.

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