Re: How to get good performance for very large lists/sets?

On 10/6/2014 3:02 AM, Richard Frith-Macdonald wrote:
> I'm wondering if anyone can help with advice on how to manage large lists/sets of items in a postgresql database.
>
> I have a database which uses multiple  lists of items roughly like this:
>
> CREATE TABLE List (
>    ID SERIAL,
>    Name VARCHAR ....
> );
>
> and a table containing individual entries in the lists:
>
> CREATE TABLE ListEntry (
>    ListID INT, /* Reference the List table */
>    ItemID INT /* References an Item table */
> ) ;
> CREATE UNIQUE INDEX ListEntryIDX ON ListEntry(ListID, ItemID);
>
> Now, there are thousands of lists, many with millions of entries, and items are added to and removed from lists in an
unpredictableway (in response to our customer's actions, not under our control).  Lists are also created by customer
actions.
>
> Finding whether a particular item is in a particular list is reasonably fast, but when we need to do things like find
allthe items in list A but not list B things can get very slow (particularly when both lists contain millions of common
items).
>
> I think that server won't use index-only scans because, even in cases where a particular list has not had any recent
changes,the ListEntry table will almost always have had some change (for one of the other lists) since its last vacuum. 
> Perhaps creating multiple ListEntry tables (one for each list) would allow better performance; but that would be
thousands(possibly tens of thousands) of tables, and allowing new tables to be created by our clients might conflict
withthings like nightly backups. 
>
> Is there a better way to manage list/set membership for many thousands of sets and many millions of items?
>
>
>


I seem to recall something about NOT IN() and nulls, but I dont recall
the details.

are you using:

select * where exists(select ...) and not exists(select ..)

or

select * where id in (select...) and id not in (select ...)



-Andy