[SQL] Atomar SQL Statement
Guys,
I want to assure that a SQL statement is atomar. It trys to check if an
email address is already found in a table, if yes, it returns the id
field of the entry. If not found, it inserts the entry with the
previously generated increment id and returns - again - the id.
My concern: in a multi threaded environment, can a second thread
interrupt this statement and eventually insert the same email address in
the table with a different id? Or is this statement atomar?
Any help would be highely appreciated.
CODE:
_id := nextval('email_id_increment');
INSERT INTO email_adr (email_id, email, lastupdate)
SELECT _id, '[EMAIL PROTECTED]', now()::timestamp
WHERE NOT EXISTS (SELECT * FROM email_adr WHERE
email='[EMAIL PROTECTED]');
- Johann
---(end of broadcast)---
TIP 6: explain analyze is your friend
Re: [SQL] Atomar SQL Statement
On Jul 7, 2006, at 7:55 , Weber, Johann (ISS Kassel) wrote: My concern: in a multi threaded environment, can a second thread interrupt this statement and eventually insert the same email address in the table with a different id? Or is this statement atomar? You're safe. Take a look at the FAQ entries on SERIAL: http://www.postgresql.org/docs/faqs.FAQ.html#item4.11.2 Hope this helps. Michael Glaesemann grzm seespotcode net ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [SQL] Atomar SQL Statement
On Fri, 7 Jul 2006, Michael Glaesemann wrote: On Jul 7, 2006, at 7:55 , Weber, Johann (ISS Kassel) wrote: My concern: in a multi threaded environment, can a second thread interrupt this statement and eventually insert the same email address in the table with a different id? Or is this statement atomar? You're safe. Take a look at the FAQ entries on SERIAL: http://www.postgresql.org/docs/faqs.FAQ.html#item4.11.2 I don't think he is, because I don't think the issue is the SERIAL behavior, but instead the NOT EXISTS behavior. Won't the NOT EXISTS in read committed potentially be true for both concurrent sessions if the second happens before the first commits, which then would mean that both sessions will go on to attempt the insert (with their own respective ids from the serial)? Without a unique constraint on email I think he can end up with the same email address with two different ids. ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [SQL] Atomar SQL Statement
On Fri, 2006-07-07 at 13:07, Stephan Szabo wrote: On Fri, 7 Jul 2006, Michael Glaesemann wrote: On Jul 7, 2006, at 7:55 , Weber, Johann (ISS Kassel) wrote: My concern: in a multi threaded environment, can a second thread interrupt this statement and eventually insert the same email address in the table with a different id? Or is this statement atomar? You're safe. Take a look at the FAQ entries on SERIAL: http://www.postgresql.org/docs/faqs.FAQ.html#item4.11.2 I don't think he is, because I don't think the issue is the SERIAL behavior, but instead the NOT EXISTS behavior. Won't the NOT EXISTS in read committed potentially be true for both concurrent sessions if the second happens before the first commits, which then would mean that both sessions will go on to attempt the insert (with their own respective ids from the serial)? Without a unique constraint on email I think he can end up with the same email address with two different ids. Yep, this is a possible race condition, if memory serves, and this is the reason for unique indexes. That way, should another transaction manage to sneak in between the two parts of this query, the unique index will still keep your data coherent. ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Fwd: [SQL] Atomar SQL Statement
On 7/7/06, Scott Marlowe [EMAIL PROTECTED] wrote: On Fri, 2006-07-07 at 13:07, Stephan Szabo wrote: On Fri, 7 Jul 2006, Michael Glaesemann wrote: On Jul 7, 2006, at 7:55 , Weber, Johann (ISS Kassel) wrote:My concern: in a multi threaded environment, can a second thread interrupt this statement and eventually insert the same email address in the table with a different id? Or is this statement atomar? You're safe. Take a look at the FAQ entries on SERIAL: http://www.postgresql.org/docs/faqs.FAQ.html#item4.11.2 I don't think he is, because I don't think the issue is the SERIAL behavior, but instead the NOT EXISTS behavior.Won't the NOT EXISTS in read committed potentially be true for both concurrent sessions if the second happens before the first commits, which then would mean that both sessions will go on to attempt the insert (with their own respective ids from the serial)? Without a unique constraint on email I think he can end up with the same email address with two different ids. Yep, this is a possible race condition, if memory serves, and this isthe reason for unique indexes.That way, should another transactionmanage to sneak in between the two parts of this query, the unique index will still keep your data coherent.I deal with these issues by first placing a unique constraint on the email field (as was suggested above) and then synchronizing the code that does the check and insert. I have also looked for instances where a unique constraint error is thrown and had the system give the user a meaningful error or responded appropriately. -Aaron
Re: [SQL] Atomar SQL Statement
On Jul 7, 2006, at 14:07 , Stephan Szabo wrote: I don't think he is, because I don't think the issue is the SERIAL behavior, but instead the NOT EXISTS behavior. I guess I should have been clearer in the issue I was addressing, which is whether or not a separate transaction could slip in and interrupt his transaction, which my understanding is that it can't. I personally would handle the insert if necessary/select in a different way, but I don't see anything necessarily wrong with his statement. I assumed that he does have a UNIQUE constraint on the email address field. I should definitely know better than to make assumption. Sorry, Johann, for not addressing your complete email, but rather just a portion of it. Michael Glaesemann grzm seespotcode net ---(end of broadcast)--- TIP 6: explain analyze is your friend
