RE: procedure using CURSOR to insert is extremely slow

[Szalontai Zoltán <szalontai.zoltan@xxxxxxxxxxx>]

Hi Milos,

 

I discuss this kind of rethinking with the team.

 

Perhaps I can copy our database on AWS for you, and you can check it.

 

thanks,

Zoltán

 

From: Milos Babic <milos.babic@xxxxxxxxx>
Sent: Thursday, April 8, 2021 8:22 PM
To: Szalontai Zoltán <szalontai.zoltan@xxxxxxxxxxx>
Cc: Pgsql Performance <pgsql-performance@xxxxxxxxxxxxxxxxxxxx>
Subject: Re: procedure using CURSOR to insert is extremely slow

 

Hi Zoltan,

 

you should try to rethink the logic behind the query.

Numerous if/then/else can be transformed into case-when, or a bunch of unions, which, I'm 100% certain will do much better than row-by-row insertion.

 

However, this is a general note.

Still doesn't explain why it takes faster to insert with deletions (?!!)

Is there any chance the set you inserting in the second run is smaller (e.g. only a fraction of the original one)?

 

If possible, you can send over a fragment of the code, and we can look into it.

 

regards,

Milos

 

 

 

 

 

 

On Thu, Apr 8, 2021 at 3:56 PM Szalontai Zoltán <szalontai.zoltan@xxxxxxxxxxx> wrote:

Hi Milos,

 

Inside the loops there are frequently if / else branches value transformations used.

We could not solve it without using a cursor.

 

Regards,

Zoltán

 

From: Milos Babic <milos.babic@xxxxxxxxx>
Sent: Thursday, April 8, 2021 2:31 PM
To: Szalontai Zoltán <szalontai.zoltan@xxxxxxxxxxx>
Cc: Pgsql Performance <pgsql-performance@xxxxxxxxxxxxxxxxxxxx>
Subject: Re: procedure using CURSOR to insert is extremely slow

 

Hi Zoltan,

 

is there any particular reason why you don't do a bulk insert as:

   insert into target_table

   select ... from source_table(s) (with joins etc)

 

Regards,

Milos

 

 

 

On Thu, Apr 8, 2021 at 1:24 PM Szalontai Zoltán <szalontai.zoltan@xxxxxxxxxxx> wrote:

Hi,

 

We have a Class db.t2.medium database on AWS.

We use a procedure to transfer data records from the Source to the Target Schema.

Transfers are identified by the log_id field in the target table.

 

The procedure is:

1 all records are deleted from the Target table with the actual log_id value

2 a complicated SELECT (numerous tables are joined) is created on the Source system

3 a cursor is defined based on this SELECT

4 we go trough the CURSOR and insert new records into the Target table with this log_id

 

(Actually we have about 100 tables in the Target schema and the size of the database backup file is about 1GByte. But we do the same for all the Target tables.)

 

Our procedure is extremely slow for the first run: 3 days for the 100 tables. For the second and all subsequent run it is fast enough (15 minutes).

The only difference between the first run and all the others is that in the first run there are no records in the Target schema with this log_id.

 

It seems, that in the first step the DELETE operation makes free some “space”, and the INSET operation in the 4. step can reuse this space. But if no records are deleted in the first step, the procedure is extremely slow.

 

To speed up the first run we found the following workaround:

We inserted dummy records into the Target tables with the proper log_id, and really the first run became very fast again.

 

Is there any “normal” way to speed up this procedure?

In the production environment there will be only “first runs”, the same log_id will never be used again.

 

 

thank

Zoltán

 

 

 

--

Milos Babic

http://www.linkedin.com/in/milosbabic

 

--

Milos Babic

http://www.linkedin.com/in/milosbabic