Обсуждение: currtid_byreloid and currtid_byrelname

In utils/adt/tid.c, there's two mysterious functions with no comments, 
and no-one calling them inside backend code AFAICT: currtid_byreloid and 
currtid_byrelname. What do they do/did?

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com

Heikki Linnakangas wrote:
> In utils/adt/tid.c, there's two mysterious functions with no comments, 
> and no-one calling them inside backend code AFAICT: currtid_byreloid 
> and currtid_byrelname. What do they do/did?
>

If you have a look at the CVS annotations ( 
http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/tid.c?annotate=1.57 
) you can see that they were introduced at version 1.12 ( 
http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/tid.c.diff?r1=1.11;r2=1.12;f=h 
)
which has the comment:

The 1st step to implement new type of scan,TidScan.
Now WHERE restriction on ctid is allowed though it is
sequentially scanned.


I often find I have to do this sort of digging to understand what's going on (or find a guilty party :-) ).

cheers

andrew

"Heikki Linnakangas" <heikki@enterprisedb.com> writes:

> In utils/adt/tid.c, there's two mysterious functions with no comments, and
> no-one calling them inside backend code AFAICT: currtid_byreloid and
> currtid_byrelname. What do they do/did?

The comments for heap_get_latest_tid() seem to apply. They're basically
wrappers that open the relation for you since that function requires it. The
real question is why you would ever have a tid without having a Relation
structure handy.

--  Gregory Stark EnterpriseDB          http://www.enterprisedb.com

Heikki Linnakangas <heikki@enterprisedb.com> writes:
> In utils/adt/tid.c, there's two mysterious functions with no comments, 
> and no-one calling them inside backend code AFAICT: currtid_byreloid and 
> currtid_byrelname. What do they do/did?

IIRC, the ODBC driver uses them, or once did, to deal with
concurrent-update situations --- it wants to be able to find the
currently committed version of a row it saw as valid earlier in the
transaction.  I think it's sort of a client-side version of the
EvalPlanQual stuff.  Hiroshi probably remembers more.
        regards, tom lane

"Tom Lane" <tgl@sss.pgh.pa.us> writes:

> Heikki Linnakangas <heikki@enterprisedb.com> writes:
>> In utils/adt/tid.c, there's two mysterious functions with no comments, 
>> and no-one calling them inside backend code AFAICT: currtid_byreloid and 
>> currtid_byrelname. What do they do/did?
>
> IIRC, the ODBC driver uses them, or once did, to deal with
> concurrent-update situations --- it wants to be able to find the
> currently committed version of a row it saw as valid earlier in the
> transaction.  I think it's sort of a client-side version of the
> EvalPlanQual stuff.  Hiroshi probably remembers more.

It could be using currtid_byreloid:
                                                     List of functions  Schema   |  Name   | Result data type |
Argumentdata types | Owner | Language |   Source code    |      Description      
 

------------+---------+------------------+---------------------+-------+----------+------------------+-----------------------pg_catalog
|currtid | tid              | oid, tid            | stark | internal | currtid_byreloid | latest tid of a tuple
 
(1 row)


But how would it use currtid_byrelname?


--  Gregory Stark EnterpriseDB          http://www.enterprisedb.com