Обсуждение: Performance hit of foreign key constraints?
I have a database which is constantly being written to. A web server's log file (and extras) is being written to it. There are no deletions or updates (at least I think so :). As the web traffic increases so will the write intensity. Right now the database tables have no foreign keys defined even though there are foreign keys. The code that inserts into the DB is simple enough (now) that we can make sure that nothing is inserted if the corresponding fk does not exist and that all fk checks pass. I want to add foreign key constraints to the table definitions but I am worried that it might be a big performance hit. Can anyone tell me how much of a performance hit adding one foreign key constraint to one field in a table will roughly be? Also, for a DB that is write-intensive and rarely read, what are some things I can do to increase performance? (Keeping in mind that there is more than on DB on the same pg server). Thanks, Jean-Christian Imbeault
On 23 Jul 2003 at 16:05, Jean-Christian Imbeault wrote: > I have a database which is constantly being written to. A web server's > log file (and extras) is being written to it. There are no deletions or > updates (at least I think so :). > > As the web traffic increases so will the write intensity. > > Right now the database tables have no foreign keys defined even though > there are foreign keys. The code that inserts into the DB is simple > enough (now) that we can make sure that nothing is inserted if the > corresponding fk does not exist and that all fk checks pass. > > I want to add foreign key constraints to the table definitions but I am > worried that it might be a big performance hit. Can anyone tell me how > much of a performance hit adding one foreign key constraint to one field > in a table will roughly be? > > Also, for a DB that is write-intensive and rarely read, what are some > things I can do to increase performance? (Keeping in mind that there is > more than on DB on the same pg server). 1. Insert them in batches. Proper size of transactions can speed the write performance heavily. 2. What kind of foreign keys you have? It might be possible to reduce FK overhead if you are checking against small number of records. 3. Tune your hardware for write performance like getting a good-for-write RAID. I forgot which performs which for read and write. 4. Tune WAL and move it to separate drive. That should win you some performance. HTH Bye Shridhar -- Beauty: What's in your eye when you have a bee in your hand.
On Wed, 23 Jul 2003, Jean-Christian Imbeault wrote: > I have a database which is constantly being written to. A web server's > log file (and extras) is being written to it. There are no deletions or > updates (at least I think so :). > > As the web traffic increases so will the write intensity. > > Right now the database tables have no foreign keys defined even though > there are foreign keys. The code that inserts into the DB is simple > enough (now) that we can make sure that nothing is inserted if the > corresponding fk does not exist and that all fk checks pass. > > I want to add foreign key constraints to the table definitions but I am > worried that it might be a big performance hit. Can anyone tell me how > much of a performance hit adding one foreign key constraint to one field > in a table will roughly be? Well, generally speaking it'll be (assuming no ref actions - and covering actions you aren't doing): one select for each insert to the table with the constraint one select for each update to the table with the constraint, in current releases unpatched one select for each update to the table with the constraint if the key is changed in patched 7.3 or 7.4beta. one select for each delete to the referenced table one select for each update to the referenced table if the key is changed plus management of the trigger queue (this can be an issue in long transactions since the queue can get big) and some misc. work in the triggers. You really want the foregin key on the table with the constraint to be indexed and using the index if you expect eitherof the referenced table conditions to happen.
On Wed, 23 Jul 2003, Stephan Szabo wrote: > > On Wed, 23 Jul 2003, Jean-Christian Imbeault wrote: > > > I have a database which is constantly being written to. A web server's > > log file (and extras) is being written to it. There are no deletions or > > updates (at least I think so :). > > > > As the web traffic increases so will the write intensity. > > > > Right now the database tables have no foreign keys defined even though > > there are foreign keys. The code that inserts into the DB is simple > > enough (now) that we can make sure that nothing is inserted if the > > corresponding fk does not exist and that all fk checks pass. > > > > I want to add foreign key constraints to the table definitions but I am > > worried that it might be a big performance hit. Can anyone tell me how > > much of a performance hit adding one foreign key constraint to one field > > in a table will roughly be? > > Well, generally speaking it'll be (assuming no ref actions - and covering > actions you aren't doing): > one select for each insert to the table with the constraint > one select for each update to the table with the constraint, in current > releases unpatched > one select for each update to the table with the constraint if the > key is changed in patched 7.3 or 7.4beta. > one select for each delete to the referenced table > one select for each update to the referenced table if the key is changed So much for answering questions before I take my shower and wake up. Make those last two be two selects, and in 7.3 and earlier, one of those selects on update to referenced happens even if the key isn't changed (there's a patch that should work to change that on -patches archive).