Обсуждение: foreign key introduces unnecessary locking ?
Hi, When two tables (table2 and table3) have foreign keys referring to a common table(table1), I am unable to have 2 concurrent transactions - one performing insert on table1 and the other on table2, when the records being inserted have the same foreign key. If I use JDBC, one of the transactions aborts. If I open 2 psql sessions and try the same, one just waits and does not show the prompt until the other transaction has been committed or aborted. For example, create table tmp1(idx int4, data int4); create table tmp2(idx2 int4, col2 int4, constraint tmpcon2 foreign key(col2) references tmp1(idx)); create table tmp3(idx3 int4, col3 int4, constraint tmpcon3 foreign key(col3) references tmp1(idx)); insert into tmp1 values(1, 1); Transaction 1 : begin work; insert into tmp2 values(2, 1); Transaction2 : begin work; insert into tmp3 values(3,1); Since such transactions are common for me, for the time-being I have dropped the foreign key constraint. Any ideas ? Rini __________________________________________________ Do You Yahoo!? Yahoo! Photos - 35mm Quality Prints, Now Get 15 Free! http://photos.yahoo.com/
Try this for both FK tables: create table tmp2(idx2 int4, col2 int4, constraint tmpcon2 foreign key(col2) references tmp1(idx) INITIALLY DEFERRED); This will defer constraint checks till transaction commit... though constraint triggers should use SnapshotDirty instead of SELECT FOR UPDATE anyway. Did you consider this, Jan? Vadim > When two tables (table2 and table3) have foreign keys > referring to a common table(table1), I am unable to > have 2 concurrent transactions - one performing insert > on table1 and the other on table2, when the records > being inserted have the same foreign key. > > If I use JDBC, one of the transactions aborts. > If I open 2 psql sessions and try the same, one just > waits and does not show the prompt until the other > transaction has been committed or aborted. > > For example, > create table tmp1(idx int4, data int4); > create table tmp2(idx2 int4, col2 int4, constraint > tmpcon2 foreign key(col2) references tmp1(idx)); > create table tmp3(idx3 int4, col3 int4, constraint > tmpcon3 foreign key(col3) references tmp1(idx)); > insert into tmp1 values(1, 1); > > Transaction 1 : > begin work; > insert into tmp2 values(2, 1); > > Transaction2 : > begin work; > insert into tmp3 values(3,1); > > Since such transactions are common for me, for the > time-being I have dropped the foreign key constraint. > Any ideas ? > > Rini > > __________________________________________________ > Do You Yahoo!? > Yahoo! Photos - 35mm Quality Prints, Now Get 15 Free! > http://photos.yahoo.com/ >
On Mon, 2 Oct 2000, Rini Dutta wrote: > When two tables (table2 and table3) have foreign keys > referring to a common table(table1), I am unable to > have 2 concurrent transactions - one performing insert > on table1 and the other on table2, when the records > being inserted have the same foreign key. > > If I use JDBC, one of the transactions aborts. > If I open 2 psql sessions and try the same, one just > waits and does not show the prompt until the other > transaction has been committed or aborted. > > For example, > create table tmp1(idx int4, data int4); > create table tmp2(idx2 int4, col2 int4, constraint > tmpcon2 foreign key(col2) references tmp1(idx)); > create table tmp3(idx3 int4, col3 int4, constraint > tmpcon3 foreign key(col3) references tmp1(idx)); > insert into tmp1 values(1, 1); > > Transaction 1 : > begin work; > insert into tmp2 values(2, 1); > > Transaction2 : > begin work; > insert into tmp3 values(3,1); > > Since such transactions are common for me, for the > time-being I have dropped the foreign key constraint. > Any ideas ? Each is attempting to grab row locks on tmp1 to prevent the rows from going away while we're testing the references. The second transaction is waiting for the row lock to go away so that it can do its row lock. I'm not sure why its failing in JDBC though.
Mikheev, Vadim wrote: > Try this for both FK tables: > > create table tmp2(idx2 int4, col2 int4, constraint > tmpcon2 foreign key(col2) references tmp1(idx) INITIALLY DEFERRED); > > This will defer constraint checks till transaction commit... > though constraint triggers should use SnapshotDirty instead of > SELECT FOR UPDATE anyway. > > Did you consider this, Jan? > > Vadim Whenever the checks are done, the transaction inserting a new reference to the key must ensure that this key cannot get deleted until it is done and it's newly inserted reference is visible to others. Otherwise a referential action, preventing referenced key deletion (or other action) wouldn't see those and it would be possibleto violate the constraint. I don't see any other way doing it than obtaining a lock. Using SnapshotDirty would mean, that one transaction could DELETE a reference, then another transaction removes the primary key (because using Dirty the DELETE is already visible), but now the first transaction rolls back. Voila, constraint violated. Jan -- #======================================================================# # It's easier to get forgiveness for being wrong than for being right. # # Let's break this rule - forgive me. # #================================================== JanWieck@Yahoo.com #
> > though constraint triggers should use SnapshotDirty instead of > > SELECT FOR UPDATE anyway. > > > > Did you consider this, Jan? > > Whenever the checks are done, the transaction inserting a new > reference to the key must ensure that this key cannot get > deleted until it is done and it's newly inserted reference is > visible to others. Otherwise a referential action, > preventing referenced key deletion (or other action) wouldn't > see those and it would be possible to violate the constraint. > > I don't see any other way doing it than obtaining a lock. > Using SnapshotDirty would mean, that one transaction could > DELETE a reference, then another transaction removes the > primary key (because using Dirty the DELETE is already > visible), but now the first transaction rolls back. Voila, > constraint violated. Using Dirty transaction removing/updating PK could see that concurrent xaction attempts to update/insert FK and so would wait for its commit/abort. Just like now same row writers wait for each other. Having this, we could insert FK without holding locks over PK. At the moment of constarint check we would see and wait concurrent PK deletion. If two xactons will wait for each other then one of them will be aborted. This behaviour is more natural for MVCC system postulated that only same-row-writers wait for each other. Why two same FK inserters should wait for each other if we can avoid this? Also, is there any way to get deferrable PK/UK constraints? I wonder why unique index is used for them. Vadim
> Using Dirty transaction removing/updating PK could see that concurrent > xaction attempts to update/insert FK and so would wait for its commit/abort. ^^^^^^^^ Of course this will require some function that would take tid as one of arguments, fetch row and check if someone is updating it. > Just like now same row writers wait for each other. Vadim