Re: foreign key locks - Mailing list pgsql-hackers

On Friday, August 31, 2012 06:59:51 AM Alvaro Herrera wrote:
> v21 is merged to latest master.
Ok, I am starting to look at this.

(working with a git merge of alvherre/fklocks into todays master)

In a very first pass as somebody who hasn't followed the discussions in the
past I took notice of the following things:

* compiles and survives make check
* README.tuplock jumps in quite fast without any introduction

* the reasons for the redesign aren't really included in the patch but just in
the - very nice - pgcon paper.

* "This is a bit of a performance regression, but since we expect FOR SHARE
locks to be seldom used, we don’t feel this is a serious problem." makes me a
bit uneasy, will look at performance separately

* Should RelationGetIndexattrBitmap check whether a unique index is referenced
from a pg_constraint row? Currently we pay part of the overhead independent
from the existance of foreign keys.

* mode == LockTupleKeyShare, == LockTupleShare, == LockTupleUpdate in
heap_lock_tuple's BeingUpdated branch look like they should be an if/else if

* I don't really like HEAP_UPDATE_KEY_REVOKED as a name, what about
HEAP_KEYS_UPDATED (akin to HEAP_HOT_UPDATED)

* how can we get infomask2 & HEAP_UPDATE_KEY_REVOKED && infomask &
HEAP_XMAX_LOCK_ONLY?

* heap_lock_tuple with mode == LockTupleKeyShare && nowait looks like it would
wait anyway in heap_lock_updated_tuple_rec

* rename HeapSatisfiesHOTUpdate, adjust comments?

* I wonder whether HeapSatisfiesHOTUpdate could be made to compute the result
for key_attrs and hot_attrs at the same time, often enough they will do the
same thing on the same column

* you didn't start it but I find that all those l$i jump labels make the code
harder to follow

* shouldn't XmaxGetUpdateXid be rather called MultiXactIdGetUpdateXid or such?

*    /*     * If the existing lock mode is identical to or weaker than the new     * one, we can act as though there is
noexisting lock and have the     * upper block handle this.     */ 
"block"?

* I am not yet sure whether the (xmax == add_to_xmax) optimization in
compute_new_xmax_infomask is actually safe

* ConditionalMultiXactIdWait and MultiXactIdWait should probably rely on a
common implementation

* I am not that convinced that moving the waiting functions away from
multixact.c is a good idea, but I guess the required knowledge about lockmodes
makes it hard not to do so

* Haven't looked yet, but I have a slightly uneasy feeling about Hot Standby
interactions. Did you look at that?

* Hm?
HeapTupleSatisfiesVacuum
#if 0            ResetMultiHintBit(tuple, buffer, xmax, true);
#endif

This obviously is not a real review, but just learning what the problem & patch
actually try to do. This is quite a bit to take in ;). I will let it sink in
ant try to have a look at the architecture and performance next.


Greetings,

Andres

.oO(and people call catalog timetravel complicated)

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
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