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

On 6 Oct 2014, at 17:54, Igor Neyman <ineyman@perceptron.com> wrote:
>
>> -----Original Message-----
>> From: pgsql-general-owner@postgresql.org [mailto:pgsql-general-owner@postgresql.org] On Behalf Of Richard
Frith-Macdonald
>> Sent: Monday, October 06, 2014 4:02 AM
>> To: pgsql-general@postgresql.org
>> Subject: [GENERAL] How to get good performance for very large lists/sets?
>>
>> 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
anunpredictable way (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
findall the items in list A but not list B things can get very slow (particularly when both lists contain millions of
commonitems). 
>>
>> 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?
>
> --
>
> You mean you are get sequential scans?
> Index-only scans are not always quicker (you could try "turning off" seq scans by setting enable_seqscan=off).
>
> Could you show your query, corresponding plans, and what don't you like about them?

I guess I didn't express myself well.

No I'm not particularly dissatisfied with any query plan;  have tried enabling/disabling different scan types to
experiment,and have been able to get better results from the query planner with such tweaks in some cases (ie with
specificdatasets), but not consistently.  Certainly the index is used quite often, and when it isn't the query planner
seemsto be making reasonable decisions. 
I've tried NOT IN, and NOT EXISTS and NOT EXISTS for different situations ...

My fundamental problem is huge datasets;  with hundreds of gigabytes of memory, I can have the lists basically in
memoryand these queries seem to be cpu-limited ... so I'm searching for a way to minimise the work the cpu has to do. 

So what I was wondering was whether this whole approach to set/list membership was the correct one to use or if there's
someother approach which can simply avoid the cpu having to look at so much data (which was why I wondered about
index-onlyscans).