On 10/6/2014 12:52 PM, Richard Frith-Macdonald wrote:
On 6 Oct 2014, at 17:54, Igor Neyman <iney...@perceptron.com> wrote:
-----Original Message-----
From: pgsql-general-ow...@postgresql.org
[mailto:pgsql-general-ow...@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 an unpredictable 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 find all the 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
with things 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 specific datasets), but not consistently. Certainly the index is used
quite often, and when it isn't the query planner seems to 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 memory and 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 some other approach which can simply
avoid the cpu having to look at so much data (which was why I wondered about
index-only scans).
Ohhh..
Um, completely left field, but, if your items are sequential in some
way, maybe there is some gross misuse of ranges you could use?
http://www.postgresql.org/docs/9.2/static/rangetypes.html
-Andy
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