Thread: How to generate unique invoice numbers for each day
Invoice numbers have format yymmddn where n is sequence number in day staring at 1 for every day. command SELECT COALESCE(MAX(nullif(substring( substring(tasudok from 7), '^[0-9]*'),'')::int),0)+1 FROM invoice where date= ?invoicedate is used to get next free invoice number if new invoice is saved. If multiple invoices are saved concurrently from different processes, they will probably get same number. How to get unique invoice number for some day in 8.1+ when multiple users create new invoices ? Andrus.
Use a sequence.
--
Jorge Godoy <jgodoy@gmail.com>
--
Jorge Godoy <jgodoy@gmail.com>
2011/1/15 Andrus Moor <kobruleht2@hot.ee>
Invoice numbers have format yymmddn
where n is sequence number in day staring at 1 for every day.
command
SELECT COALESCE(MAX(nullif(substring( substring(tasudok from 7), '^[0-9]*'),'')::int),0)+1
FROM invoice
where date= ?invoicedate
is used to get next free invoice number if new invoice is saved.
If multiple invoices are saved concurrently from different processes, they will probably get same number.
How to get unique invoice number for some day in 8.1+ when multiple users create new invoices ?
Andrus.
--
Sent via pgsql-general mailing list (pgsql-general@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-general
There are 365 days in year.
Do you really think pre-creating sequence for every day for every year is best solution ?
Andrus.
----- Original Message -----From: Jorge GodoyTo: Andrus MoorSent: Saturday, January 15, 2011 8:41 PMSubject: ***SPAM*** Re: [GENERAL] How to generate unique invoice numbers foreach dayUse a sequence.
--
Jorge Godoy <jgodoy@gmail.com>2011/1/15 Andrus Moor <kobruleht2@hot.ee>Invoice numbers have format yymmddn
where n is sequence number in day staring at 1 for every day.
command
SELECT COALESCE(MAX(nullif(substring( substring(tasudok from 7), '^[0-9]*'),'')::int),0)+1
FROM invoice
where date= ?invoicedate
is used to get next free invoice number if new invoice is saved.
If multiple invoices are saved concurrently from different processes, they will probably get same number.
How to get unique invoice number for some day in 8.1+ when multiple users create new invoices ?
Andrus.
--
Sent via pgsql-general mailing list (pgsql-general@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-general
Why would you do that?
You can always reset the sequence at the end of the day.
--
Jorge Godoy <jgodoy@gmail.com>
--
Jorge Godoy <jgodoy@gmail.com>
On Sat, Jan 15, 2011 at 17:09, Andrus Moor <kobruleht2@hot.ee> wrote:
There are 365 days in year.Do you really think pre-creating sequence for every day for every year is best solution ?Andrus.----- Original Message -----From: Jorge GodoyTo: Andrus MoorSent: Saturday, January 15, 2011 8:41 PMSubject: ***SPAM*** Re: [GENERAL] How to generate unique invoice numbers foreach dayUse a sequence.
--
Jorge Godoy <jgodoy@gmail.com>2011/1/15 Andrus Moor <kobruleht2@hot.ee>Invoice numbers have format yymmddn
where n is sequence number in day staring at 1 for every day.
command
SELECT COALESCE(MAX(nullif(substring( substring(tasudok from 7), '^[0-9]*'),'')::int),0)+1
FROM invoice
where date= ?invoicedate
is used to get next free invoice number if new invoice is saved.
If multiple invoices are saved concurrently from different processes, they will probably get same number.
How to get unique invoice number for some day in 8.1+ when multiple users create new invoices ?
Andrus.
--
Sent via pgsql-general mailing list (pgsql-general@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-general
If the gaps (user gets a number from a sequence and then rollbacks the transaction) are not a problem, then the sequences (reset every day) are probably the best solution. If the gaps are a problem (which is usually the case with invoicing systems), then you need to manage that on your own, e.g. using a table with one row for sequence (but that might be a bottleneck with multiple users and a lot of invoices). Tomas Dne 15.1.2011 20:32, Jorge Godoy napsal(a): > Why would you do that? > > You can always reset the sequence at the end of the day. > > -- > Jorge Godoy <jgodoy@gmail.com <mailto:jgodoy@gmail.com>> > > > On Sat, Jan 15, 2011 at 17:09, Andrus Moor <kobruleht2@hot.ee > <mailto:kobruleht2@hot.ee>> wrote: > > There are 365 days in year. > Do you really think pre-creating sequence for every day for every > year is best solution ? > > Andrus. > > ----- Original Message ----- > *From:* Jorge Godoy <mailto:jgodoy@gmail.com> > *To:* Andrus Moor <mailto:kobruleht2@hot.ee> > *Cc:* pgsql-general@postgresql.org > <mailto:pgsql-general@postgresql.org> > *Sent:* Saturday, January 15, 2011 8:41 PM > *Subject:* ***SPAM*** Re: [GENERAL] How to generate unique > invoice numbers foreach day > > Use a sequence. > > -- > Jorge Godoy <jgodoy@gmail.com <mailto:jgodoy@gmail.com>> > > > 2011/1/15 Andrus Moor <kobruleht2@hot.ee <mailto:kobruleht2@hot.ee>> > > Invoice numbers have format yymmddn > > where n is sequence number in day staring at 1 for every day. > > command > > SELECT COALESCE(MAX(nullif(substring( substring(tasudok from > 7), '^[0-9]*'),'')::int),0)+1 > FROM invoice > where date= ?invoicedate > > is used to get next free invoice number if new invoice is saved. > > If multiple invoices are saved concurrently from different > processes, they will probably get same number. > > How to get unique invoice number for some day in 8.1+ when > multiple users create new invoices ? > > Andrus. > > -- > Sent via pgsql-general mailing list > (pgsql-general@postgresql.org > <mailto:pgsql-general@postgresql.org>) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-general > > >
Invoices can entered also some days forward or back. Users enters invoice date and expected program to generate next sequential number for this day.
Different users can enter invoices for different days.
Andrus.
----- Original Message -----From: Jorge GodoyTo: Andrus MoorSent: Saturday, January 15, 2011 9:32 PMSubject: Re: [GENERAL] How to generate unique invoice numbers foreach dayWhy would you do that?You can always reset the sequence at the end of the day.
--
Jorge Godoy <jgodoy@gmail.com>On Sat, Jan 15, 2011 at 17:09, Andrus Moor <kobruleht2@hot.ee> wrote:There are 365 days in year.Do you really think pre-creating sequence for every day for every year is best solution ?Andrus.----- Original Message -----From: Jorge GodoyTo: Andrus MoorSent: Saturday, January 15, 2011 8:41 PMSubject: ***SPAM*** Re: [GENERAL] How to generate unique invoice numbers foreach dayUse a sequence.
--
Jorge Godoy <jgodoy@gmail.com>2011/1/15 Andrus Moor <kobruleht2@hot.ee>Invoice numbers have format yymmddn
where n is sequence number in day staring at 1 for every day.
command
SELECT COALESCE(MAX(nullif(substring( substring(tasudok from 7), '^[0-9]*'),'')::int),0)+1
FROM invoice
where date= ?invoicedate
is used to get next free invoice number if new invoice is saved.
If multiple invoices are saved concurrently from different processes, they will probably get same number.
How to get unique invoice number for some day in 8.1+ when multiple users create new invoices ?
Andrus.
--
Sent via pgsql-general mailing list (pgsql-general@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-general
In that case you have to manage the IDs on your own, the sequences won't help you in this (unless you really create one sequence for each day, which does not seem like a good solution to me). A really simple solution might be to do a BEFORE INSERT trigger that checks the last ID inserted for the day, locks that ID somehow (e.g. the record with the ID) so that you don't get duplicate IDs and uses that ID. Another solution is to manage the IDs on your own in a separate table, i.e. create a table CREATE TABLE invoice_numbers ( invoice_date DATE NOT NULL PRIMARY KEY, invoice_last INTEGER NOT NULL DEFAULT 0 ); and then something like BEGIN; UPDATE invoice_numbers SET invoice_last = (invoice_last + 1) WHERE invoice_date = '2010-11-...' RETURNING invoice_last; ... use the new invoice_last value COMMIT; But again, this might be a concurrency bottleneck if there's a lot of invoices created at the same time by different users (or if the transaction is long). Tomas Dne 15.1.2011 21:04, Andrus Moor napsal(a): > Invoices can entered also some days forward or back. Users enters > invoice date and expected program to generate next sequential number for > this day. > Different users can enter invoices for different days. > > Andrus. > > > ----- Original Message ----- > *From:* Jorge Godoy <mailto:jgodoy@gmail.com> > *To:* Andrus Moor <mailto:kobruleht2@hot.ee> > *Cc:* pgsql-general@postgresql.org > <mailto:pgsql-general@postgresql.org> > *Sent:* Saturday, January 15, 2011 9:32 PM > *Subject:* Re: [GENERAL] How to generate unique invoice numbers > foreach day > > Why would you do that? > > You can always reset the sequence at the end of the day. > > -- > Jorge Godoy <jgodoy@gmail.com <mailto:jgodoy@gmail.com>> > > > On Sat, Jan 15, 2011 at 17:09, Andrus Moor <kobruleht2@hot.ee > <mailto:kobruleht2@hot.ee>> wrote: > > There are 365 days in year. > Do you really think pre-creating sequence for every day for > every year is best solution ? > > Andrus. > > ----- Original Message ----- > *From:* Jorge Godoy <mailto:jgodoy@gmail.com> > *To:* Andrus Moor <mailto:kobruleht2@hot.ee> > *Cc:* pgsql-general@postgresql.org > <mailto:pgsql-general@postgresql.org> > *Sent:* Saturday, January 15, 2011 8:41 PM > *Subject:* ***SPAM*** Re: [GENERAL] How to generate unique > invoice numbers foreach day > > Use a sequence. > > -- > Jorge Godoy <jgodoy@gmail.com <mailto:jgodoy@gmail.com>> > > > 2011/1/15 Andrus Moor <kobruleht2@hot.ee > <mailto:kobruleht2@hot.ee>> > > Invoice numbers have format yymmddn > > where n is sequence number in day staring at 1 for every > day. > > command > > SELECT COALESCE(MAX(nullif(substring( substring(tasudok > from 7), '^[0-9]*'),'')::int),0)+1 > FROM invoice > where date= ?invoicedate > > is used to get next free invoice number if new invoice > is saved. > > If multiple invoices are saved concurrently from > different processes, they will probably get same number. > > How to get unique invoice number for some day in 8.1+ > when multiple users create new invoices ? > > Andrus. > > -- > Sent via pgsql-general mailing list > (pgsql-general@postgresql.org > <mailto:pgsql-general@postgresql.org>) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-general > > >
On Sat, 15 Jan 2011, Andrus Moor wrote: > There are 365 days in year. Do you really think pre-creating sequence for > every day for every year is best solution ? Andrus, I just saw this thread so my idea may not work for you. What I'd do is use the Julian date (that is, the sequential day from 1-365/6) followed by 1, 2, or 3 digits depending on how many invoices are generated each day. Makes sorting and comparisons simple. Rich
On 2011-01-15, Andrus Moor <kobruleht2@hot.ee> wrote: > Invoice numbers have format yymmddn > > where n is sequence number in day staring at 1 for every day. > > command > > SELECT COALESCE(MAX(nullif(substring( substring(tasudok from 7), > '^[0-9]*'),'')::int),0)+1 > FROM invoice > where date= ?invoicedate > > is used to get next free invoice number if new invoice is saved. > > If multiple invoices are saved concurrently from different processes, they > will probably get same number. > > How to get unique invoice number for some day in 8.1+ when multiple users > create new invoices ? use a sequence, reset the sequence each night. -- ⚂⚃ 100% natural
This is an very common and well understood problem. Take a look at this: http://www.varlena.com/GeneralBits/130.php -- Regards, Peter Geoghegan
Dne 16.1.2011 03:29, Jasen Betts napsal(a): > On 2011-01-15, Andrus Moor <kobruleht2@hot.ee> wrote: >> Invoice numbers have format yymmddn >> >> where n is sequence number in day staring at 1 for every day. >> >> command >> >> SELECT COALESCE(MAX(nullif(substring( substring(tasudok from 7), >> '^[0-9]*'),'')::int),0)+1 >> FROM invoice >> where date= ?invoicedate >> >> is used to get next free invoice number if new invoice is saved. >> >> If multiple invoices are saved concurrently from different processes, they >> will probably get same number. >> >> How to get unique invoice number for some day in 8.1+ when multiple users >> create new invoices ? > > use a sequence, reset the sequence each night. This was already proposed but he can't do that as the users may issue invoices for different days (not just for 'today'). Which means the user may create an invoice for monday, then another one for tuesday and then another one for monday again. A single sequence can't solve this. So either he has to create a sequence for each day, but he'll get a lot of sequences (and it's a bit messy), and there will be gaps (not sure if this is acceptable). Or he can use the solution proposed in the General Bits 130 (which is basically the solution I've already proposed) and maintain gapless sequences using a table. Tomas
On 01/15/2011 12:13 PM, Andrus Moor wrote: > Invoice numbers have format yymmddn > > where n is sequence number in day staring at 1 for every day. > > command > > SELECT COALESCE(MAX(nullif(substring( substring(tasudok from 7), '^[0-9]*'),'')::int),0)+1 > FROM invoice > where date= ?invoicedate > > is used to get next free invoice number if new invoice is saved. > > If multiple invoices are saved concurrently from different processes, they will probably get same number. > > How to get unique invoice number for some day in 8.1+ when multiple users create new invoices ? > > Andrus. > I understand this is a business rule, and you cant change it. My opinion is you are doing it wrong. Don't encode more than one bit of data into one column. You have two bits of data,date and invoice number. Use TWO columns! Let the invoice number auto-inc and never reset it. You want to find invoiceson some day, query the date field. You want to find a specific invoice, query the invoice number. Dont combinethe two. Why write a bunch of bug ridden code that encodes and decodes the two fields into one? But I know... you cant change it. Someone up the chain thinks its just the bee's knees. I had to say it though. Its beensaid. I will go in peace. -Andy
Andy, >> SELECT COALESCE(MAX(nullif(substring( substring(tasudok from 7), >> '^[0-9]*'),'')::int),0)+1 >> FROM invoice >> where date= ?invoicedate >> >> is used to get next free invoice number if new invoice is saved. >> >> If multiple invoices are saved concurrently from different processes, >> they will probably get same number. > I understand this is a business rule, and you cant change it. Yes. This is customer requirement and I cannot change it. Is it reasonable/how to implement the following: 1. plpgsql command obtains some lock 2. It uses SELECT COALESCE(MAX(nullif(substring( substring(tasudok from 7), '^[0-9]*'),'')::int),0)+1 FROM invoice where date= ?invoicedate to get next number for invoice date day 3. It adds new invoice with this numbder to database 4. It releases the lock. Or is it better to maintain separate sequence or separate table of free numbers for every day ? using sequence approach: Application checks for sequence name in form invoiceno_yyyymmdd if sequence does not exist it is created. For concurrent adding second caller gets sequence exists exception and in this case this query can re-executed. Next value is obtained from sequence sequneces older than 1 year should be purded automatically since max 1 year backward numbers may created. Which is best solution ? using new record approach: 1. lock table 2. in this day sequnece does notr exist, add it 3. get next value for day, increment number in this table 4. unlock the table. Which command should be used to obtain exclusise write access to table (lock some reosurce or semaphore) ? Andrus.
On 01/16/2011 11:00 AM, Andrus Moor wrote: > Andy, > >>> SELECT COALESCE(MAX(nullif(substring( substring(tasudok from 7), '^[0-9]*'),'')::int),0)+1 >>> FROM invoice >>> where date= ?invoicedate >>> >>> is used to get next free invoice number if new invoice is saved. >>> >>> If multiple invoices are saved concurrently from different processes, they will probably get same number. > >> I understand this is a business rule, and you cant change it. > > Yes. This is customer requirement and I cannot change it. OR... can you go back to your customer and tell them they wont like this. Really really they should let you do it correctly. I find people dont change because they dont have to, not because there is an actual reason. Many times, givena description of how hard and how messy something will be to code, I have convinced people that a simple business changeand simple code is really the best approach. But I have hit walls. Things I could not change, but I did try. > Is it reasonable/how to implement the following: Sorry, I have no idea. -Andy
>> Yes. This is customer requirement and I cannot change it. > OR... can you go back to your customer and tell them they wont like this. > Really really they should let you do it correctly. I find people dont > change because they dont have to, not because there is an actual reason. > Many times, given a description of how hard and how messy something will > be to code, I have convinced people that a simple business change and > simple code is really the best approach. But I have hit walls. Things I > could not change, but I did try. My Visual FoxPro application works OK in this case. I used FLOCK() to lock invoice header table (FLOCK() waits indefinitely until lock is obtained and reads fresh data from disk), used SELECT MAX( CAST( SUBSTRING(invoiceno,8) AS INT ) )+1 FROM invoices WHERE date= m.invoice_date to get next free number, inserted invoice and unlocked the table. Customer expects Postgres to be more powerful than FoxPro . He don't understand why this stops working after upgrade. Andrus.
On 16 Jan 2011, at 18:56, Andrus Moor wrote: > My Visual FoxPro application works OK in this case. > I used FLOCK() to lock invoice header table (FLOCK() waits indefinitely until lock is obtained and reads fresh data fromdisk), > > used > > SELECT MAX( CAST( SUBSTRING(invoiceno,8) AS INT ) )+1 > FROM invoices > WHERE date= m.invoice_date > > to get next free number, inserted invoice and unlocked the table. If you really need gapless sequences, then you would do something rather similar in Postgres. Instead of the whole database file you only lock the file for the table containing the sequence. That's called serialization,something you can't get around if you require gapless sequences. The documentation does a much better job atexplaining serialization than I could, I'm sure. That's also why people were suggesting ways around that requirement, as it's a costly feature with regards to database performance. There are blogs about how to create gapless sequences in Postgres, so I won't go into detail about them. > Customer expects Postgres to be more powerful than FoxPro . He don't understand why this stops working after upgrade. And he is right too! To the customer the database is probably that thing that they put their data in and that does all themagic stuff for them. To you it's that thing that you implement all that magic stuff in! Half of what the customer callshis database is what you implemented. Your customer doesn't care about details like that though, he cares about a working system. The database (the part that wecall the database) does it's thing rather well in 99.99% of the cases - usually when the database (as seen from the customer'spoint of view) doesn't do what it's supposed to do, the problem is in the business logic that you put in that database. So to summarise: What's failing her isn't the database, it's you. Where you failed, heck, I don't know... You probably didn't get all the requirements right, or you forgot to test this particularpart of the application, or whatever. Don't fuss about it too much though, it's almost impossible to not fail somewherewith complicated applications like this. Consider it a lesson learned. Just don't blame the database for it, especially not on a mailing-list about that database ;) Alban Hertroys -- If you can't see the forest for the trees, cut the trees and you'll see there is no forest. !DSPAM:737,4d333dbf11702139115944!
I will sugest to: 1. Delete point 1. 2. In point 2. add FOR UPDATE 3. Use READ COMMITED TRANSACTION ISOLATION LEVEL Don't lock tables, You wrote you can generate invoices for few days backward, so you don't need locking whole table. Don't use seqences, as sequence value will don't get back when transaction is rolled back (You need to prevent gaps). Locking with UPDATE, or FOR UPDATE is much more portable. If you generate invoices in massive operation, probably when process runs no one will be able to create invoice, but you don't need to create multi thread application. In any approach preventing gaps, locking is required. This is real life situation; imagine you have two coworkers and then they need to create invoices, so they looks in ledger (or a last day copy of ledger in their offices; international company, but no Internet, only fax and telephone) and checks last number used, what should be done next? "Andrus Moor" <kobruleht2@hot.ee> Sunday 16 January 2011 18:00:58 > Andy, > > >> SELECT COALESCE(MAX(nullif(substring( substring(tasudok from 7), > >> '^[0-9]*'),'')::int),0)+1 > >> FROM invoice > >> where date= ?invoicedate > >> > >> is used to get next free invoice number if new invoice is saved. > >> > >> If multiple invoices are saved concurrently from different processes, > >> they will probably get same number. > > > > I understand this is a business rule, and you cant change it. > > Yes. This is customer requirement and I cannot change it. > Is it reasonable/how to implement the following: > > 1. plpgsql command obtains some lock > 2. It uses > > SELECT COALESCE(MAX(nullif(substring( substring(tasudok from 7), > '^[0-9]*'),'')::int),0)+1 > FROM invoice > where date= ?invoicedate > > to get next number for invoice date day > > 3. It adds new invoice with this numbder to database > > 4. It releases the lock. > > Or is it better to maintain separate sequence or separate table of free > numbers for every day ? >
Thank you. > 2. In point 2. add FOR UPDATE > 3. Use READ COMMITED TRANSACTION ISOLATION LEVEL > > Don't lock tables, You wrote you can generate invoices for few days > backward, > so you don't need locking whole table. > > Don't use seqences, as sequence value will don't get back when transaction > is > rolled back (You need to prevent gaps). > > Locking with UPDATE, or FOR UPDATE is much more portable. > > If you generate invoices in massive operation, probably when process runs > no > one will be able to create invoice, but you don't need to create multi > thread > application. > In any approach preventing gaps, locking is required. This is real life > situation; imagine you have two coworkers and then they need to create > invoices, so they looks in ledger (or a last day copy of ledger in their > offices; international company, but no Internet, only fax and telephone) > and > checks last number used, what should be done next? Using read commited isolation level requires knowing in start of transaction will it perform new invoice adding or not. This requires changing program logic a lot. Currently script which creates day seq numbers runs inside transaction . Transaction starter does not know will special isolation required or not. Changing blindly all transactions to use this isolation level decreases perfomance and may lead to deadlocks. In my case I can assume that transaction newer fails since business rules are verified and this is simple insert (inrare cases if it fails due to disk failure etc then gaps are allowed). Can this knowledge used to create simpler solution ? Andrus.
Dne 16.1.2011 22:44, Andrus Moor napsal(a): > Thank you. > >> 2. In point 2. add FOR UPDATE >> 3. Use READ COMMITED TRANSACTION ISOLATION LEVEL >> >> Don't lock tables, You wrote you can generate invoices for few days >> backward, >> so you don't need locking whole table. >> >> Don't use seqences, as sequence value will don't get back when >> transaction >> is >> rolled back (You need to prevent gaps). >> >> Locking with UPDATE, or FOR UPDATE is much more portable. >> >> If you generate invoices in massive operation, probably when process runs >> no >> one will be able to create invoice, but you don't need to create multi >> thread >> application. > >> In any approach preventing gaps, locking is required. This is real life >> situation; imagine you have two coworkers and then they need to create >> invoices, so they looks in ledger (or a last day copy of ledger in their >> offices; international company, but no Internet, only fax and telephone) >> and >> checks last number used, what should be done next? > > Using read commited isolation level requires knowing in start of > transaction will it perform new invoice adding or not. This requires > changing program logic a lot. > Currently script which creates day seq numbers runs inside transaction . > Transaction starter does not know will special isolation required or not. > Changing blindly all transactions to use this isolation level decreases > perfomance and may lead to deadlocks. I really am not sure what you mean by this. The isolation levels are implemented in the database, you don't need to change the application at all. And there are only two isolation levels in PostgreSQL - READ COMMITTED and SERIALIZABLE, where the READ COMMITTED is the less restrictive one (and default). So everything runs (at least) in READ COMMITTED mode, no matter what you do. You don't need to change anything. Yes, locking may in some cases lead to deadlocks, that's true. For example creating several invoices (for different days) in a single transaction may lead to a deadlock. But that's a feature, not a bug. And you can get around this by creating all the invoices in the same order (e.g. sorted by date) - this prevents deadlocks. > In my case I can assume that transaction newer fails since business > rules are verified and this is simple insert (inrare cases if it fails > due to disk failure etc then gaps are allowed). > Can this knowledge used to create simpler solution ? Locking when updating the very same value is inevitable. If you update the same row from two sessions, one of them has to wait until the other one commits or rolls back. You can't get around this in a transactional environment. In Oracle you could solve this using an autonomous transaction, but there's nothing like that in PostgreSQL. So if you don't want to use the approach proposed in General Bits 130 (the one with gapless sequences implemented using a table), the only option I'm aware of is to create one sequence for each day. Tomas
Tomas Vondra wrote on 16.01.2011 23:41: > Yes, locking may in some cases lead to deadlocks, that's true. For > example creating several invoices (for different days) in a single > transaction may lead to a deadlock. But that's a feature, not a bug. Hmm, a single transaction cannot deadlock itself as far as I know. A deadlock can only happen between two different transactions (T1 locks R1, waits for R2, T2 locks R2 waits for R1) Regards Thomas
mail@smogura.eu (Radosław Smogura) writes: > In any approach preventing gaps, locking is required. This is real life > situation; imagine you have two coworkers and then they need to create > invoices, so they looks in ledger (or a last day copy of ledger in their > offices; international company, but no Internet, only fax and telephone) and > checks last number used, what should be done next? In this case, I don't imagine it's necessary to do terribly much explicit locking. The crucial thing to 'serialize' is the "ownership" of the unique invoice number, and a PRIMARY KEY constraint does that perfectly nicely, and with downsides that are pretty much unavoidable. - In the simple, no-conflict case, the value is generated by querying the highest value for the specified day, adding 1 to it, and in the face of no conflicting requests going in at the same time, having everything succeed. - On the other hand, if two workers are trying to generate invoices concurrently, both compute the highest value, adding 1, both getting the same value. The first worker clicks on whatever does the COMMIT, the work succeeds, and their invoice is successfully stored. The second worker selects whatever does the COMMIT, and, since an invoice is already committed with the same invoice number, the request fails based on the violation of the primary key. The solution to that problem is to resubmit the request, querying for a new invoice number, which (hopefully!) succeeds. And there's not much possible as an alternative to this resubmission. If this all is turning out badly, then I suggest a third possibility, namely to: - Initially use a sequence (or similar) to generate an invoice number in that is sure to be unique, but which doesn't conform to expectations. Suppose... We set up a sequence, and initially assign invoice ID values based purely on that sequence. As it contains no '-' characters, it never conflicts with the IDs that we *want*. And as increased values are assigned automatically, two workers entering data never fight over specific values. - Then, have a single process that comes in afterwards and rewrites the invoice IDs in accordance with policy. In effect, this process looks for all invoices where the ID is purely numeric. And it renumbers them into the format desired. Since this renumbering takes place within a single process, it doesn't "fight" with other processes that might be holding onto those not-yet-properly-formatted IDs. The "magic" still needed is to run that rewrite process. Something needs to invoke it periodically to fix up the IDs. Left as exercise to the reader ;-). -- let name="cbbrowne" and tld="gmail.com" in name ^ "@" ^ tld;; "Seriously, I think you really need to find out who took your sense of humor, beat the crap out of him, and take it back. It's getting old." -- Michael J Ash <mikeash@csd.uwm.edu>, on comp.lang.objective-c