Re: Update on true serializable techniques in MVCC

[ Forgot the list, resending. ]

2009/12/16 Boszormenyi Zoltan <zb(at)cybertec(dot)at>:

> Robert Haas írta:
>
>> On Wed, Dec 16, 2009 at 1:25 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>>
>>> On Wed, Dec 16, 2009 at 1:14 PM, Alvaro Herrera
>>> <alvherre(at)commandprompt(dot)com> wrote:
>>>
>>>> So you'd have to disable HOT updates when true serializability was
>>>> active?
>>>
>>> I thought about that, but I don't think so.   HOT only applies to
>>> updates, and predicate locking only applies to inserts.  Unless I have
>>> my head in the sand?
>>
>> Err, no, wait.  Predicate locking can apply to updates, but since HOT
>> updates never update an indexed column, I think we might still be OK?
>
> A predicate can include columns from an index plus others.
> Am I missing something?

This whole concept ("next-key locking") also applies in case there are
no indexes. In the case of a table scan, the "next key" is either the
next row relative to the scanned range (if the DBMS supports the
notion of non-full table scans, for example if the table contents are
themselves stored in sorted order), or something that indicates that
the whole table was scanned (i.e., a table lock).

Therefore, with next-key locking you better don't have too many table
scans if you want to have any concurrent transactions.

Nicolas