Re: locks in CREATE TRIGGER, ADD FK

Neil Conway wrote:
> So I think it should be possible to lock both the heap relation and the 
> index with ExclusiveLock, which would allow SELECTs on them. This would 
> apply to both the single relation and multiple relation variants of 
> CLUSTER (since we do each individual clustering in its own transaction).
> 
> ... except that when we rebuild the relation's indexes, we acquire an 
> AccessExclusiveLock on the index. This would introduce the risk of 
> deadlock. It seems necessary to acquire an AccessExclusiveLock when 
> rebuilding shared indexes, since we do the index build in-place, but I 
> think we can get by with an ExclusiveLock in the non-shared case, for 
> similar reasons as above: we build the new index and then swap relfilenodes.

Certainly we need to upgrade to an exclusive table lock to replace the
heap table.  Do we want to get a shared lock and possibly starve waiting
for an exclusive lock on the table to swap the new one in?  Do we do
such escallation anywhere else?

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