Thread: Improving performance of select query

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Karthik Shivashankar schrieb am 14.12.2020 um 12:38:
> I have a postgres(v9.5) table named customer holding 1 billion rows.
> It is not partitioned but it has an index against the primary key
> (integer). I need to keep a very few records (say, about 10k rows)
> and remove everything else.
>
> /insert into customer_backup select * from customer where customer_id in (<id1>,<id2>,..); /
>
>  
>
> If I go for something like above I'm afraid the insert-select may take a very long time as when I ran
>
> /select count(*) from customer;/
>
> it is taking about 45 minutes to return the count.

Well, you need to compare the time with the same condition you use in your
CREATE TABLE .. AS SELECT statement,

e.g.:

   select count(*)
   from customer
   where id in (....);

Or:

   explain (analyze)
   select *
   from customer
   where id in (....);


Regards
Thomas

> On Dec 14, 2020, at 4:47 AM, Thomas Kellerer <shammat@gmx.net> wrote:
>
> Karthik Shivashankar schrieb am 14.12.2020 um 12:38:
>> I have a postgres(v9.5) table named customer holding 1 billion rows.
>> It is not partitioned but it has an index against the primary key
>> (integer). I need to keep a very few records (say, about 10k rows)
>> and remove everything else.
>>
>> /insert into customer_backup select * from customer where customer_id in (<id1>,<id2>,..); /
>>
>>
>>
>> If I go for something like above I'm afraid the insert-select may take a very long time as when I ran
>>
>> /select count(*) from customer;/
>>
>> it is taking about 45 minutes to return the count.
>
> Well, you need to compare the time with the same condition you use in your
> CREATE TABLE .. AS SELECT statement,
>
> e.g.:
>
>   select count(*)
>   from customer
>   where id in (....);
>
> Or:
>
>   explain (analyze)
>   select *
>   from customer
>   where id in (....);
>
>
> Regards
> Thomas
>
As for the actually copy of the specific records, I would ‘where exists’ (even possibly with a temp table of ids)
ratherthan in(id1..id10000) 

>