lastval()

Dennis Bjorklund <db@zigo.dhs.org> writes:

Here is a small patch that implements a function lastval() that
works just like currval() except that it give the current
value of the last sequence used by nextval().

Why is that a good idea? In a complex application it'd be awfully easy
to break logic that depends on such a thing.

regards, tom lane

Tom Lane wrote:

Why is that a good idea? In a complex application it'd be awfully easy
to break logic that depends on such a thing.

True, but I think it offers a usefully concise syntax for simpler
applications. Perhaps the documentation should be amended to mention the
potential risks? (e.g. additional nextval() calls in between the
nextval() you are interested in and the lastval()).

-Neil

Neil Conway <neilc@samurai.com> writes:

Tom Lane wrote:

Why is that a good idea? In a complex application it'd be awfully easy
to break logic that depends on such a thing.

True, but I think it offers a usefully concise syntax for simpler
applications. Perhaps the documentation should be amended to mention the
potential risks?

Like, say, the sequence being deleted before the lastval call?

If I thought it was a good idea at all, I'd bother to criticize the
patch itself --- it's got some problems.

regards, tom lane

On Sun, 8 May 2005, Tom Lane wrote:

Why is that a good idea? In a complex application it'd be awfully easy
to break logic that depends on such a thing.

Of course it can break. currval() can also break in a complex application
with triggers and rules that do things the developer does not expect.

There are however lots of cases where it is safe and useful. Not the least
when you want to port an application that uses similar features.

--
/Dennis Bj�rklund

On Sun, 8 May 2005, Tom Lane wrote:

Like, say, the sequence being deleted before the lastval call?

Then you get an error message. Same thing if you have revoked the rights
on the sequence before you call lastval().

In this case you can get a value that belong to a sequence that is
deleted. Is that better? To me it's a sign that something is wrong with
the application and an error is better to get. It's not like it's hard to
store a int64 value instead. It's in fact simpler, but I just don't see
that it solve any problem. If anything it can hide problems.

If you want lastval() to work just don't delete the sequence. It's as
simple as that.

The thing is that I don't care how it's implemented, it's the feature
itself that is more importent to decide if we want it or not. I'm sure the
code can be fixed so everybody is happy it in the end,

--
/Dennis Bj�rklund

On Mon, 9 May 2005, Dennis Bjorklund wrote:

The thing is that I don't care how it's implemented, it's the feature
itself that is more importent to decide if we want it or not. I'm sure the
code can be fixed so everybody is happy it in the end,

You could implement this on top of the current nextval without backend
changes.

Create a wrapper function on top of nextval that stores the value in a
temp table. Or a session variable if your PL language of choice has
them.

lastval would do a select on the temp table.

- Heikki

Heikki Linnakangas wrote:

On Mon, 9 May 2005, Dennis Bjorklund wrote:

The thing is that I don't care how it's implemented, it's the feature
itself that is more importent to decide if we want it or not. I'm
sure the
code can be fixed so everybody is happy it in the end,

You could implement this on top of the current nextval without backend
changes.

Create a wrapper function on top of nextval that stores the value in a
temp table. Or a session variable if your PL language of choice has them.

lastval would do a select on the temp table.

And this is making life easier for anybody? I don't think so.

cheers

andrew

Dennis Bjorklund wrote:

Here is a small patch that implements a function lastval() that
works just like currval() except that it give the current
value of the last sequence used by nextval().

What do people think of this idea? (Tom seems opposed, I'm just
wondering if there are other opinions out there.)

I like the concept, but I haven't looked at the code -- I'd be happy to
review the implementation, although I won't waste my time if most people
are opposed to the idea itself.

-Neil

Neil Conway wrote:

Dennis Bjorklund wrote:

Here is a small patch that implements a function lastval() that
works just like currval() except that it give the current
value of the last sequence used by nextval().

What do people think of this idea? (Tom seems opposed, I'm just
wondering if there are other opinions out there.)

I like the concept, but I haven't looked at the code -- I'd be happy to
review the implementation, although I won't waste my time if most people
are opposed to the idea itself.

I can't speak to the code but lastval is something that has been
requested by my customers many times.

Sincerely,

Joshua D. Drake

Neil Conway wrote:

Dennis Bjorklund wrote:

Here is a small patch that implements a function lastval() that
works just like currval() except that it give the current
value of the last sequence used by nextval().

What do people think of this idea? (Tom seems opposed, I'm just
wondering if there are other opinions out there.)

I like the idea of lastval, though I would rather see us just use
currval() with no argument for it, rather than invent a new function
name. It does the same as currval('last sequence called') right?

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

I like the concept, but I haven't looked at the code -- I'd be happy to
review the implementation, although I won't waste my time if most people
are opposed to the idea itself.

It'd make implementing various PHP userland functions a real breeze...

Chris

Abhijit Menon-Sen said:

At 2005-05-11 10:55:37 +1000, neilc@samurai.com wrote:

Here is a small patch that implements a function lastval() [...]

What do people think of this idea? (Tom seems opposed, I'm just
wondering if there are other opinions out there.)

For what it's worth, I think it's a bad idea.

In the MySQL wire protocol (hi Dennis!), the "last insert id" is sent
along with every "OK" message, and the client can just keep the value
in memory. Users call a function to retrieve that value, rather than
issuing a "SELECT nextval()".

You can do both - they have an SQL level function as well as supporting it
at the protocol layer. See
http://dev.mysql.com/doc/mysql/en/information-functions.html

So the server-side lastval() function is not enough for any meaningful
compatibility. The client would also need to be changed to provide the
pgsql_last_insert_id() or a similar function (which could do a "SELECT
lastval()" internally).

In this situation -- where both client changes AND a server round-trip
are required -- what's the point of adding cruft to the server? Might
as well confine changes to the client, and use nextval to implement the
feature.

I don't believ it can be sensibly done by the client alone. Either it needs
something like this or it shouldn't be done at all.

By the way, what would lastval() do if an insert trigger inserts a row
into a table with another serial column?

or more than one? Yes, it's not good in certain circumstances. That doesn't
make it useless in all circumstances.

I'm not jumping out of my seat to have this. But as Joshua points out, it is
frequently requested.

cheers

andrew

At 2005-05-11 10:55:37 +1000, neilc@samurai.com wrote:

Here is a small patch that implements a function lastval() [...]

What do people think of this idea? (Tom seems opposed, I'm just
wondering if there are other opinions out there.)

For what it's worth, I think it's a bad idea.

In the MySQL wire protocol (hi Dennis!), the "last insert id" is sent
along with every "OK" message, and the client can just keep the value
in memory. Users call a function to retrieve that value, rather than
issuing a "SELECT nextval()".

So the server-side lastval() function is not enough for any meaningful
compatibility. The client would also need to be changed to provide the
pgsql_last_insert_id() or a similar function (which could do a "SELECT
lastval()" internally).

In this situation -- where both client changes AND a server round-trip
are required -- what's the point of adding cruft to the server? Might
as well confine changes to the client, and use nextval to implement
the feature.

By the way, what would lastval() do if an insert trigger inserts a row
into a table with another serial column?

-- ams

Abhijit Menon-Sen wrote:

At 2005-05-11 10:55:37 +1000, neilc@samurai.com wrote:

Here is a small patch that implements a function lastval() [...]

What do people think of this idea? (Tom seems opposed, I'm just
wondering if there are other opinions out there.)

For what it's worth, I think it's a bad idea.

In the MySQL wire protocol (hi Dennis!), the "last insert id" is sent
along with every "OK" message, and the client can just keep the value
in memory. Users call a function to retrieve that value, rather than
issuing a "SELECT nextval()".

So the server-side lastval() function is not enough for any meaningful
compatibility. The client would also need to be changed to provide the
pgsql_last_insert_id() or a similar function (which could do a "SELECT
lastval()" internally).

In this situation -- where both client changes AND a server round-trip
are required -- what's the point of adding cruft to the server? Might
as well confine changes to the client, and use nextval to implement
the feature.

By the way, what would lastval() do if an insert trigger inserts a row
into a table with another serial column?

It fails, just like it would fail now if the trigger inserted into the
same table that used the trigger, or a rule.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

At 2005-05-10 23:30:05 -0400, pgman@candle.pha.pa.us wrote:

By the way, what would lastval() do if an insert trigger inserts
a row into a table with another serial column?

It fails, just like it would fail now if the trigger inserted into
the same table that used the trigger, or a rule.

I don't understand what you mean. "Just like it would fail now"? It
doesn't exist yet, how can it fail? And how would it know when to
fail anyway, rather than return a wrong value?

-- ams

Abhijit Menon-Sen wrote:

At 2005-05-10 23:30:05 -0400, pgman@candle.pha.pa.us wrote:

By the way, what would lastval() do if an insert trigger inserts
a row into a table with another serial column?

It fails, just like it would fail now if the trigger inserted into
the same table that used the trigger, or a rule.

I don't understand what you mean. "Just like it would fail now"? It
doesn't exist yet, how can it fail? And how would it know when to
fail anyway, rather than return a wrong value?

Uh, if the table's sequence name is 'tab_x_seq', and you do
currval('tab_x_seq'), you will get the trigger or rule insert id in that
case.

So, currval() widens a problem we already have if the rule/trigger
inserts into the same table.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

Bruce Momjian <pgman@candle.pha.pa.us> writes:

Abhijit Menon-Sen wrote:

By the way, what would lastval() do if an insert trigger inserts a row
into a table with another serial column?

It fails, just like it would fail now if the trigger inserted into the
same table that used the trigger, or a rule.

If it actually *failed* that would be one thing, but the proposed
patch does not do that. It looks more like the philosophy we usually
denigrate MySQL for, viz never fail even if you are staring a certain
application bug in the face.

regards, tom lane

Dennis Bjorklund wrote:

+ Datum
+ lastval(PG_FUNCTION_ARGS)
+ {
+ 	Relation	seqrel;
+ 	int64		result;
+ 
+ 	if (last_used_seq == NULL) {
+ 		ereport(ERROR,
+ 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+ 				 errmsg("nextval have not been used in the current session")));
+ 	}

"has not been" would be more better English.

+ 
+ 	seqrel = relation_open(last_used_seq->relid, NoLock);
+ 
+ 	acquire_share_lock (seqrel, last_used_seq);
+ 
+ 	if (pg_class_aclcheck(last_used_seq->relid, GetUserId(), ACL_SELECT) != ACLCHECK_OK)
+ 		ereport(ERROR,
+ 				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+ 				 errmsg("permission denied for sequence with OID %d",
+ 						last_used_seq->relid)));

"%d" is always the wrong formatting sequence for OIDs (they are
unsigned, hence %u). But in any case user-visible error messages should
specify the name of the sequence, which you can get via
RelationGetRelationName(seqrel)

+ 
+ 	if (last_used_seq->increment == 0)	/* nextval/read_info were not called */
+ 		ereport(ERROR,
+ 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+ 				 errmsg("currval of sequence with OID %d is not yet defined in this session",
+ 						last_used_seq->relid)));

See above; however, when will this error actually be invoked? (The
comment is wrong, as last_used_seq won't be defined if nextval has not
been called.)

/*
+  * If we haven't touched the sequence already in this transaction,
+  * we need to acquire AccessShareLock.  We arrange for the lock to
+  * be owned by the top transaction, so that we don't need to do it
+  * more than once per xact.
+  */
+ static void
+ acquire_share_lock (Relation seqrel,
+ 					SeqTableData *data)

Confusing SeqTable and SeqTableData * is bad style. I personally don't
like putting pointers into typedefs, but since the PG code does this,
SeqTable should be used consistently rather than SeqTableData *. The
same applies to the definition of "last_used_seq".

Comments on behavior:

neilc=# select setval('foo', 500);
setval
--------
500
(1 row)

neilc=# select lastval();
lastval
---------
500
(1 row)

I'm not sure it's necessarily _wrong_ to update lastval() on both setval
and nextval, but if that's the behavior we're going to implement, it
should surely be documented.

neilc=# create sequence bar ; select nextval ('bar') ; drop sequence bar;
CREATE SEQUENCE
nextval
---------
1
(1 row)

DROP SEQUENCE
neilc=# select lastval();
ERROR: XX000: could not open relation with OID 16389

Needs a friendlier error message.

-Neil

On Sun, 2005-05-08 at 19:00 +0200, Dennis Bjorklund wrote:

Here is a small patch that implements a function lastval() that
works just like currval() except that it give the current
value of the last sequence used by nextval().

Have you had a chance to respin this patch per my earlier comments on
the implementation, Dennis?

-Neil