Re: Optimizing bulk update performance

On Fri, Apr 26, 2013 at 9:41 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Yang Zhang <yanghatespam(at)gmail(dot)com> writes:
>> It currently takes up to 24h for us to run a large set of UPDATE
>> statements on a database, which are of the form:
>
>> UPDATE table SET field1 = constant1, field2 = constant2, ... WHERE
>> id = constid
>
>> (We're just overwriting fields of objects identified by ID.)
>
> Forgive the obvious question, but you do have an index on "id", right?
> Have you checked it's being used (ie EXPLAIN ANALYZE on one of these)?

Totally valid question. That is the primary key with its own index.
Yes, we verified that explain says it just use a simple index scan.
Each individual query runs reasonably quickly (we can run several
dozen such statements per second).

>
>> The tables have handfuls of indices each and no foreign key constraints.
>
> How much is a "handful"?

The table with the largest volume of updates (our bottleneck) has four indexes:

"account_pkey" PRIMARY KEY, btree (id)
"account_createddate" btree (createddate)
"account_id_prefix" btree (id text_pattern_ops)
"account_recordtypeid" btree (recordtypeid)

>
>> It takes 2h to import a `pg_dump` of the entire DB. This seems like a
>> baseline we should reasonably target.
>
> Well, maybe. You didn't say what percentage of the DB you're updating.

It can be 10-50% of rows changed - a large portion.

>
> But the thing that comes to mind here is that you're probably incurring
> a network round trip for each row, and maybe a query-planning round as
> well, so you really can't expect that this is going to be anywhere near
> as efficient as a bulk load operation. You could presumably get rid of
> the planner overhead by using a prepared statement. Cutting the network
> overhead is going to require a bit more ingenuity --- could you move
> some logic into a stored procedure, perhaps, so that one command from
> the client is sufficient to update multiple rows?

You're right, we're only sequentially issuing (unprepared) UPDATEs.

If we ship many UPDATE statements per call to our DB API's execution
function (we're using Python's psycopg2 if that matters, but I think
that just binds libpq), would that avoid the network round trip per
statement?

If not, what if we use anonymous procedures (DO) to run multiple
UPDATE statements?

Finally, we could use the technique highlighted in my third bullet and
use COPY (or at least multiple-value INSERT), then merging the new
data with the old. Would that be the most direct route to maximum
performance?

In any case, I assume deleting and rebuilding indexes is important
here, yes? But what about raising checkpoint_segments - does this
actually help sustained throughput?

>
> regards, tom lane

--
Yang Zhang
http://yz.mit.edu/