Thread: predicate locking knowledge

Attached is a patch for the documentation.  We currently state that we
don't know of any production DBMS which uses predicate locking.  Current
versions of at least two commercial products (Sybase ASE and Microsoft
SQL Server) do use it.  I believe this has been true since 1990 or
earlier.

If people need documentation for this, I can pull something together.
If you want quick confirmation, and you have access to either of the
above products, just try the test case from the documentation page this
patch modifies.  Briefly, in these products no page is ever read or
written without first acquiring a lock; in SERIALIZABLE mode all locks
are held until COMMIT or ROLLBACK.

Stating that we're not aware of it doesn't enhance our credibility with
people using these products, who can easily confirm that the "contrived"
example in our documentation does indeed generate a serialization error
in their current product.  There is also a chance, however slim, that we
could lead someone into trouble with this statement.

-Kevin

>>> On Thu, Mar 23, 2006 at  1:13 pm, in message
<44229EE6.EE98.0025.0@wicourts.gov>, "Kevin Grittner"
<Kevin.Grittner@wicourts.gov> wrote:

Attachment didn't seem to go through.  Embedding below.


Index: mvcc.sgml
===================================================================
RCS file: /projects/cvsroot/pgsql/doc/src/sgml/mvcc.sgml,v
retrieving revision 2.55
diff -c -r2.55 mvcc.sgml
*** mvcc.sgml    10 Mar 2006 19:10:48 -0000    2.55
--- mvcc.sgml    23 Mar 2006 18:58:13 -0000
***************
*** 471,477 ****
      result in problems.  (Certainly the example above is rather
contrived
      and unlikely to represent real software.)  Accordingly,
      <productname>PostgreSQL</productname> does not implement
predicate
!     locking, and so far as we are aware no other production DBMS does
either.
     </para>

     <para>
--- 471,477 ----
      result in problems.  (Certainly the example above is rather
contrived
      and unlikely to represent real software.)  Accordingly,
      <productname>PostgreSQL</productname> does not implement
predicate
!     locking.
     </para>

     <para>

"Kevin Grittner" <Kevin.Grittner@wicourts.gov> writes:
> Attached is a patch for the documentation.  We currently state that we
> don't know of any production DBMS which uses predicate locking.  Current
> versions of at least two commercial products (Sybase ASE and Microsoft
> SQL Server) do use it.

Are they actually doing fully-general predicate locks (able to cope with
arbitrary predicate conditions), or merely index range locks?  I don't
mind expanding the docs, but the context of the statement is the cost of
doing general predicate locking, and pointing to DBMSes that do a subset
case doesn't make that statement invalid.

            regards, tom lane