Обсуждение: Last inserted row id with complex PK

Hi Nelson:

On Wed, Jan 8, 2014 at 4:54 PM, Nelson Green <nelsongreen84@gmail.com> wrote:
> I have a projects log table with a three column PK, project_num, person_num,
> and sequence, where each new entry for a project/person combination
> increments the sequence, which is not an auto incrementing sequence. Is
> there any way to retrieve the last entry to the table? For instance, if the
> last entry for person 427 on project 15 was sequence number 125, is that
> information available to me anywhere?

SELECT MAX(sequence) FROM project_log_table WHERE person_num=427 AND
project_num=15

If it's a PK it should have an index and I believe it will be quite fast.

AAMOF, you could even build the above with max+1 into the insert query
for a new entry and have the values returned using a RETURNING clause.

Francisco Olarte.

Francisco Olarte <folarte@peoplecall.com> writes:
> Hi Nelson:
> On Wed, Jan 8, 2014 at 4:54 PM, Nelson Green <nelsongreen84@gmail.com> wrote:
>> I have a projects log table with a three column PK, project_num, person_num,
>> and sequence, where each new entry for a project/person combination
>> increments the sequence, which is not an auto incrementing sequence. Is
>> there any way to retrieve the last entry to the table? For instance, if the
>> last entry for person 427 on project 15 was sequence number 125, is that
>> information available to me anywhere?

> SELECT MAX(sequence) FROM project_log_table WHERE person_num=427 AND
> project_num=15

Note that this will not work terribly well if there are concurrent
insertions for the same person/project.  If that's not an issue, though,
it should be fine.  It may be OK even if there are sometimes concurrent
insertions, if you are prepared to retry duplicate-key failures.

> If it's a PK it should have an index and I believe it will be quite fast.

It will be fast as long as sequence is the low-order column in the index.

            regards, tom lane

Nelson Green wrote
> My apologies, I was not completely clear. I will not know any of the
> columns in advance. The most recent insert is the result of user input
> from
> a web form, so I won't know what project or what user generated the last
> insert. That was why I wandered if that information was stored anywhere in
> the system.

I'd probably add either (or both) a table-level auto-sequence field and a
"recordcreationdate" default timestamptz field.

David J.





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On 08 Jan 2014, at 16:54, Nelson Green <nelsongreen84@gmail.com> wrote:

> I have a projects log table with a three column PK, project_num, person_num, and sequence, where each new entry for a
project/personcombination increments the sequence, which is not an auto incrementing sequence. Is there any way to
retrievethe last entry to the table? For instance, if the last entry for person 427 on project 15 was sequence number
125,is that information available to me anywhere? 

I think the answer rather depends on what you mean with this last inserted row and for what purpose you need it.

If you want that information right after it’s been inserted (for example because you need that information in an FK
relation),you can use INSERT..RETURNING to return the values of the relevant PK fields. 

If that’s not what you’re after, then what is it that determines which record is the “last one”? If you can’t identify
sucha record from your data while you need that information, then something is missing from your model. 

If you’re planning to add such information to your model based on your current data, you might be able to get somewhat
reliableresults looking at the transaction xid’s that the records were created in. There are a number of pitfalls to
thatapproach though, the most obvious one being transaction xid wraparound. Of course, multiple inserts from the same
transactionwould (originally) have the same xid, so you wouldn’t be able to determine which one of those would be the
latest(unless they’re for the same person/project, obviously). 
Such information could then be used to add a field with, for example, an incrementing sequence.

Alban Hertroys
--
If you can't see the forest for the trees,
cut the trees and you'll find there is no forest.

On Wed, Jan 8, 2014 at 4:54 PM, Nelson Green <nelsongreen84@gmail.com> wrote:
> I have a projects log table with a three column PK, project_num, person_num,
> and sequence, where each new entry for a project/person combination
> increments the sequence, which is not an auto incrementing sequence. Is
> there any way to retrieve the last entry to the table? For instance, if the
> last entry for person 427 on project 15 was sequence number 125, is that
> information available to me anywhere?

SELECT MAX(sequence) FROM project_log_table WHERE person_num=427 AND
project_num=15

Hi Francisco,

My apologies, I was not completely clear. I will not know any of the columns in advance. The most recent insert is the result of user input from a web form, so I won't know what project or what user generated the last insert. That was why I wandered if that information was stored anywhere in the system.

-- 
john r pierce                                      37N 122W
somewhere on the middle of the left coast

Hi Nelson:

On Wed, Jan 8, 2014 at 7:14 PM, Nelson Green <nelsongreen84@gmail.com> wrote:
> My apologies, I was not completely clear. I will not know any of the columns
> in advance. The most recent insert is the result of user input from a web
> form, so I won't know what project or what user generated the last insert.
> That was why I wandered if that information was stored anywhere in the
> system.

You mean someone already inserted a (project, user, sequence) tuple
and you need it? So the problem of calculating the last sequence for a
given ( p, u ) combination has been solved by the inserting web form
and you just need to communicate the result to another process ?

It seems what you need is an auxiliary log table ( serial + (P,U,S) )
which could be managed by a trigger, or made the web form do the
modifications using a procedure. Anyway, a lot more detail will be
needed to solve this.

Francisco Olarte.