On 12/8/2014 10:30 AM, Andy Colson wrote:
> On 12/7/2014 9:31 PM, Daniel Begin wrote:
>> I have just completed the bulk upload of a large database. Some tables
>> have billions of records and no constraints or indexes have been applied
>> yet. About 0.1% of these records may have been duplicated during the
>> upload and I need to remove them before applying constraints.
>>
>> I understand there are (at least) two approaches to get a table without
>> duplicate records…
>>
>> - Delete duplicate records from the table based on an
>> appropriate select clause;
>>
>> - Create a new table with the results from a select distinct
>> clause, and then drop the original table.
>>
>> What would be the most efficient procedure in PostgreSQL to do the job
>> considering …
>>
>> - I do not know which records were duplicated;
>>
>> - There are no indexes applied on tables yet;
>>
>> - There is no OIDS on tables yet;
>>
>> - The database is currently 1TB but I have plenty of disk
>> space.
>>
>> Daniel
>>
>
> How would you detect duplicate? Is there a single field that would be
> duplicated? Or do you have to test a bunch of different fields?
>
> If its a single field, you could find dups in a single pass of the table
> with:
>
> create index bigtable_key on bigtable(key);
> select key, count(*) from bigtable group by key having count(*) > 1;
>
> Save that list, and decide on some way of deleting the dups.
>
> The index might help the initial select, but will really help re-query
> and delete statements.
>
> -Andy
>
>
I just thought of a more generic way.
1) make a non-unique index on bigtable
2) make a temp table
3) -- copy only dups
insert into temp table
select * from big table where (its a duplicate);
4)
delete from bigtable where keys in (select key from temp);
5)
insert into bigtable
select distinct from temp;
This would minimize the amount of data you have to move around. Depends
on how hard step 3 is to write. Index not required but would help both
step 3 and 4 be faster.
-Andy