Question

After searching stackoverflow.com I found several questions asking how to remove duplicates, but none of them addressed speed.

In my case I have a table with 10 columns that contains 5 million exact row duplicates. In addition, I have at least a million other rows with duplicates in 9 of the 10 columns. My current technique is taking (so far) 3 hours to delete these 5 million rows. Here is my process:

-- Step 1:  **This step took 13 minutes.** Insert only one of the n duplicate rows into a temp table
select
    MAX(prikey) as MaxPriKey, -- identity(1, 1)
    a,
    b,
    c,
    d,
    e,
    f,
    g,
    h,
    i
into #dupTemp
FROM sourceTable
group by
    a,
    b,
    c,
    d,
    e,
    f,
    g,
    h,
    i
having COUNT(*) > 1

Next,

-- Step 2: **This step is taking the 3+ hours**
-- delete the row when all the non-unique columns are the same (duplicates) and
-- have a smaller prikey not equal to the max prikey
delete 
from sourceTable
from sourceTable
inner join #dupTemp on  
    sourceTable.a = #dupTemp.a and
    sourceTable.b = #dupTemp.b and
    sourceTable.c = #dupTemp.c and
    sourceTable.d = #dupTemp.d and
    sourceTable.e   = #dupTemp.e and
    sourceTable.f = #dupTemp.f and
    sourceTable.g = #dupTemp.g and
    sourceTable.h = #dupTemp.h and
    sourceTable.i   = #dupTemp.i and
    sourceTable.PriKey != #dupTemp.MaxPriKey  

Any tips on how to speed this up, or a faster way? Remember I will have to run this again for rows that are not exact duplicates.

Thanks so much.

UPDATE:
I had to stop step 2 from running at the 9 hour mark.
I tried OMG Ponies’ method and it finished after only 40 minutes.
I tried my step 2 with Andomar’s batch delete, it ran the 9 hours before I stopped it.
UPDATE:
Ran a similar query with one less field to get rid of a different set of duplicates and the query ran for only 4 minutes (8000 rows) using OMG Ponies’ method.

I will try the cte technique the next chance I get, however, I suspect OMG Ponies’ method will be tough to beat.

Answer 1

What about EXISTS:

DELETE FROM sourceTable
 WHERE EXISTS(SELECT NULL
                FROM #dupTemp dt
               WHERE sourceTable.a = dt.a 
                 AND sourceTable.b = dt.b 
                 AND sourceTable.c = dt.c 
                 AND sourceTable.d = dt.d 
                 AND sourceTable.e = dt.e 
                 AND sourceTable.f = dt.f 
                 AND sourceTable.g = dt.g 
                 AND sourceTable.h = dt.h 
                 AND sourceTable.i = dt.i 
                 AND sourceTable.PriKey < dt.MaxPriKey)