Delete duplicate records with no change in between

I have a products table where I insert around 150,000 records a day. Most of them are redundant, but I need to keep them because of the new expiration date. I get product feeds from about 5 out of 30 vendors a day. Each vendor has around 35,000 unique products. No product can belong to more than one vendor.

CREATE TABLE vendor_prices (
id serial PRIMARY KEY,
vendor integer NOT NULL,
sku character varying(25) NOT NULL,
category_name character varying(100) NOT NULL,
price numeric(8,5) NOT NULL,
effective_date timestamp without time zone,
expiration_date timestamp without time zone DEFAULT (now() + '1 year'::interval)
);

I'm trying to delete irrelevant records where there was no price change and it's no longer the last update for said product, eg.:

effective_date price
'2015-05-01' $1.99
'2015-05-02' $1.99 delete
'2015-05-03' $1.59
'2015-05-04' $1.99
'2015-05-05' $1.99 delete
'2015-05-06' $1.99 keep for new expiration date

So after each load (I figured it'll be easier for one vendor at a time) I want to do some kind of delete. Here is the long non-performing solution that I came up with.

```
CREATE OR REPLACE FUNCTION remove_vendor_price_dupes(_vendor integer)
RETURNS integer AS
$BODY$
BEGIN
-- Delete Redundant prices
delete from vendor_prices
where id in (
select id from (
select vp1.id, vp1.vendor, vp1.sku, vp1.price, vp1.effective_date, vp1.expiration_date
from vendor_prices vp1
inner join (
select vendor, sku, price from vendor_prices
where vendor = _vendor
group by vendor, sku, price
) vp2
on vp1.vendor = vp2.vendor and vp1.sku = vp2.sku and vp1.price = vp2.price
where vp1.vendor = _vendor
) dupe

-- fetch the irrelevant record
WHERE (select a.effective_date from vendor_prices a
where vendor = _vendor
and a.price = dupe.price and

Core feature is the window function lag().

Also pay special attention to avoid deadlocks and race conditions with concurrent deletes and inserts (which can affect which rows to delete!):

CREATE OR REPLACE FUNCTION remove_vendor_price_dupes(_vendor int)
  RETURNS integer AS
$func$
DECLARE
   del_ct int;
BEGIN
   -- this may or may not be necessary:
   -- lock rows to avoid race conditions with concurrent deletes
   PERFORM 1
   FROM   vendor_prices
   WHERE  vendor = _vendor
   ORDER  BY sku, effective_date, id  -- guarantee row locks in consistent order
   FOR    UPDATE;

   -- delete redundant prices
   DELETE FROM vendor_prices v
   USING (
      SELECT id
           , price = lag(price) OVER w  -- same as last row
             AND (lead(id) OVER w) IS NOT NULL AS del  -- not last row
      FROM   vendor_prices
      WHERE  vendor = _vendor
      WINDOW w AS (PARTITION BY sku ORDER BY effective_date, id)
      ) d
   WHERE v.id = d.id
   AND   d.del;

   GET DIAGNOSTICS del_ct = ROW_COUNT;  -- optional:
   RETURN del_ct;  -- return number of deleted rows
END
$func$  LANGUAGE plpgsql;

Call:

SELECT remove_vendor_price_dupes(1);

Notes

-
The current version of the 9.3 major release is 9.3.6. The project recommends that ...


all users run the latest available minor release for whatever major version is in use.

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A multicolumn index on (vendor, sku, effective_date, id) would be perfect for this - in this particular order. But Postgres can combine indexes rather efficiently, too.

It might pay to add the otherwise irrelevant price as last item ot the index to get index-only scans out of this. You'll have to test.

-
Since you have concurrent deletes it may be a good idea to run a separate delete per vendor to reduce the potential for race conditions and deadlocks. Since there are only a few vendors, this seems like a reasonable partitioning. (Many tiny calls would be comparatively slow.)

-
I am running a separate SELECT (PERFORM in plpgsql, since we do not use the result) because the row locking clause FOR UPDATE cannot be used together with window functions. Don't let the keyword mislead you, this is not just for updates. I am locking all rows for the given vendor, since the result depends on all rows. Concurrent reads are not impaired, only concurrent writes have to wait until we are done. That's another reason why deleting rows for one vendor at a time in a separate transaction should be best.

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sku is unique per product, so we can PARTITION BY it.

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ORDER BY effective_date, id: your first version of the question included code for duplicate rows, so I added id to ORDER BY as additional tie breaker. This way it works for duplicates on (sku, effective_date) as well.

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To preserve the last row for each set: AND (lead(id) OVER w) IS NOT NULL. Reusing the same window for lead() is cheap - independent of the added explicit WINDOW clause - that's just syntax shorthand for convenience.

-
I am locking rows in the same order: ORDER BY sku, effective_date, id. Make sure that concurrent DELETEs operate in the same order to avoid deadlocks. If all other transactions delete no more than a single row within the same transaction, there cannot be deadlocks and you don't need the row locking at all.

-
If concurrent INSERTs could lead to a different result (make different rows obsolete), you have to lock the whole table in EXCLUSIVE mode instead to avoid race conditions:

LOCK TABLE vendor_prices IN EXCLUSIVE MODE;

Do that only if it's necessary. It blocks all concurrent write access.

-
I am returning the number of rows deleted, but that's totally optional. You might as well return nothing and declare the function as RETURNS void.

StackExchange Database Administrators Q#101844, answer score: 3