Re: Need help understanding pg_locks

Florian Pflug wrote:
> On Jul11, 2011, at 17:31 , Bruce Momjian wrote:
> > Tom Lane wrote:
> >> Florian Pflug <fgp@phlo.org> writes:
> >>> On Jul11, 2011, at 17:11 , Tom Lane wrote:
> >>>> Yeah, I think this patch is going in the wrong direction altogether.
> >>>> It would be better to modify the description of virtualtransaction
> >>>> and pid to say that those are the "locking" entity.
> >>
> >>> Hm, we already kinda of say that. Both descriptions include the phrase
> >>> "... holding or awaiting this lock.". The column "mode" says
> >>> "... held or desired by this process", which I guess is similar enough
> >>> to make it clear that these are related.
> >>
> >>> Its the columns which refer to the locked object which simply say
> >>> "object", and thus leave it open if that means locked or a locking.
> >>
> >>> Could we split that table in two parts, one for the fields referring
> >>> to the locked object and one for the locking entity, or does that depart
> >>> too far from the way we document other system catalogs and views?
> >>
> >> Then you'd have to join them, which would not be an improvement from
> >> anybody's standpoint.
> >>
> >> Maybe we could just add a paragraph above the "pg_locks Columns" table
> >> that says explicitly that virtualtransaction and pid describe the entity
> >> holding or awaiting the lock, and the others describe the object being
> >> locked?  Any way you slice it, putting this information into the
> >> per-column table is going to be repetitive.
> >
> > Frankly, whenever anyone says "object", they might as well call it
> > "thing".  It seems to be a content-less word.  Maybe just replace the
> > word "object" with "lock".
>
> I like that, as long as we make it ".. lock is/isn't *on* a ...", and not
> just "... lock is/isn't a". After all, the lock very clearly isn't a
> relation or xid or whatever - it's a, well, lock.
>
> We'd then have
>   OID of the database in which the lock exists, or zero if the lock is on a
>   shared object, or null if the lock is on a transaction ID.
>
>   OID of the relation, or null if the lock is not on a relation or part of a
>   relation.
>
>   ...
>
>   ID of a transaction, or null if the lock is not on a transaction ID

OK, I went with this wording, using "lock object is on" terminology.
Applied patch attached --- adjustments welcomed.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
new file mode 100644
index d4a1d36..c5851af
*** a/doc/src/sgml/catalogs.sgml
--- b/doc/src/sgml/catalogs.sgml
***************
*** 6930,6936 ****
        <entry>
         OID of the database in which the object exists, or
         zero if the object is a shared object, or
!        null if the object is a transaction ID
        </entry>
       </row>
       <row>
--- 6930,6936 ----
        <entry>
         OID of the database in which the object exists, or
         zero if the object is a shared object, or
!        null if the lock object is on a transaction ID
        </entry>
       </row>
       <row>
***************
*** 6938,6945 ****
        <entry><type>oid</type></entry>
        <entry><literal><link linkend="catalog-pg-class"><structname>pg_class</structname></link>.oid</literal></entry>
        <entry>
!        OID of the relation, or null if the object is not
!        a relation or part of a relation
        </entry>
       </row>
       <row>
--- 6938,6945 ----
        <entry><type>oid</type></entry>
        <entry><literal><link linkend="catalog-pg-class"><structname>pg_class</structname></link>.oid</literal></entry>
        <entry>
!        OID of the relation, or null if the lock object is not
!        on a relation or part of a relation
        </entry>
       </row>
       <row>
***************
*** 6947,6954 ****
        <entry><type>integer</type></entry>
        <entry></entry>
        <entry>
!        Page number within the relation, or null if the object
!        is not a tuple or relation page
        </entry>
       </row>
       <row>
--- 6947,6954 ----
        <entry><type>integer</type></entry>
        <entry></entry>
        <entry>
!        Page number within the relation, or null if the lock object
!        is not on a tuple or relation page
        </entry>
       </row>
       <row>
***************
*** 6956,6962 ****
        <entry><type>smallint</type></entry>
        <entry></entry>
        <entry>
!        Tuple number within the page, or null if the object is not a tuple
        </entry>
       </row>
       <row>
--- 6956,6963 ----
        <entry><type>smallint</type></entry>
        <entry></entry>
        <entry>
!        Tuple number within the page, or null if the lock object is not
!        on a tuple
        </entry>
       </row>
       <row>
***************
*** 6964,6971 ****
        <entry><type>text</type></entry>
        <entry></entry>
        <entry>
!        Virtual ID of a transaction, or null if the object is not a
!        virtual transaction ID
        </entry>
       </row>
       <row>
--- 6965,6972 ----
        <entry><type>text</type></entry>
        <entry></entry>
        <entry>
!        Virtual ID of a transaction lock, or null if the lock object is not
!        on a virtual transaction ID
        </entry>
       </row>
       <row>
***************
*** 6973,6979 ****
        <entry><type>xid</type></entry>
        <entry></entry>
        <entry>
!        ID of a transaction, or null if the object is not a transaction ID
        </entry>
       </row>
       <row>
--- 6974,6980 ----
        <entry><type>xid</type></entry>
        <entry></entry>
        <entry>
!        ID of a transaction lock, or null if the lock object is not on a transaction ID
        </entry>
       </row>
       <row>
***************
*** 6982,6988 ****
        <entry><literal><link linkend="catalog-pg-class"><structname>pg_class</structname></link>.oid</literal></entry>
        <entry>
         OID of the system catalog containing the object, or null if the
!        object is not a general database object
        </entry>
       </row>
       <row>
--- 6983,6989 ----
        <entry><literal><link linkend="catalog-pg-class"><structname>pg_class</structname></link>.oid</literal></entry>
        <entry>
         OID of the system catalog containing the object, or null if the
!        lock object is not on a general database object.
        </entry>
       </row>
       <row>
***************
*** 6991,6997 ****
        <entry>any OID column</entry>
        <entry>
         OID of the object within its system catalog, or null if the
!        object is not a general database object.
         For advisory locks it is used to distinguish the two key
         spaces (1 for an int8 key, 2 for two int4 keys).
        </entry>
--- 6992,6998 ----
        <entry>any OID column</entry>
        <entry>
         OID of the object within its system catalog, or null if the
!        lock object is not on a general database object.
         For advisory locks it is used to distinguish the two key
         spaces (1 for an int8 key, 2 for two int4 keys).
        </entry>
***************
*** 7004,7010 ****
         For a table column, this is the column number (the
         <structfield>classid</> and <structfield>objid</> refer to the
         table itself).  For all other object types, this column is
!        zero.  Null if the object is not a general database object
        </entry>
       </row>
       <row>
--- 7005,7011 ----
         For a table column, this is the column number (the
         <structfield>classid</> and <structfield>objid</> refer to the
         table itself).  For all other object types, this column is
!        zero.  Null if the lock object is not on a general database object.
        </entry>
       </row>
       <row>