Re: [HACKERS] Solution to UPDATE...INSERT problem
"Christopher Kings-Lynne" <[EMAIL PROTECTED]> writes: >> Uh, why exactly do you think this is race-free? > How about: > INSERT INTO table SELECT 1, 'foo' WHERE NOT EXISTS (SELECT TRUE FROM table > WHERE pkcol=1 FOR UPDATE); > It's a lot more straightforward and has a FOR UPDATE. Can this still cause > unique constraint failures? Certainly. FOR UPDATE locks an existing row; it cannot lock the condition of non-existence of a row. regards, tom lane ---(end of broadcast)--- TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED]
Re: [HACKERS] Solution to UPDATE...INSERT problem
> Uh, why exactly do you think this is race-free? > > It looks fancy, but AFAICS the SELECT will return info that is correct > as of its starting timestamp; which is not enough to guarantee that the > INSERT won't conflict with another transaction doing the same thing > concurrently. How about: INSERT INTO table SELECT 1, 'foo' WHERE NOT EXISTS (SELECT TRUE FROM table WHERE pkcol=1 FOR UPDATE); It's a lot more straightforward and has a FOR UPDATE. Can this still cause unique constraint failures? Chris ---(end of broadcast)--- TIP 2: you can get off all lists at once with the unregister command (send "unregister YourEmailAddressHere" to [EMAIL PROTECTED])
Re: [HACKERS] Solution to UPDATE...INSERT problem
"Christopher Kings-Lynne" <[EMAIL PROTECTED]> writes: > INSERT INTO table SELECT 1, 'foo' EXCEPT SELECT 1, 'foo' FROM table WHERE > pkcol=1; > See? So now that INSERT statement will insert the row if it doesn't exist, > or insert zero rows if it does. You are then guaranteed that your > transaction will not fail and rollback, so you can repeat your update, or do > the insert first and then the update, etc. Uh, why exactly do you think this is race-free? It looks fancy, but AFAICS the SELECT will return info that is correct as of its starting timestamp; which is not enough to guarantee that the INSERT won't conflict with another transaction doing the same thing concurrently. regards, tom lane ---(end of broadcast)--- TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED]
