RE: procedure using CURSOR to insert is extremely slow

How to check execution plans?

We are in the Loop of the Cursor, and we do insert operations in it.

From: Hervé Schweitzer (HER) <herve(dot)schweitzer(at)dbi-services(dot)com>
Sent: Thursday, April 8, 2021 1:40 PM
To: Szalontai Zoltán <szalontai(dot)zoltan(at)t-online(dot)hu>;
pgsql-performance(at)lists(dot)postgresql(dot)org
Subject: Re: procedure using CURSOR to insert is extremely slow

If you do a delete on the first step without any statistics, you request
will do a full scan of the table, which will be slower.

Did you check the different execution plans ?

_____

From: Szalontai Zoltán <szalontai(dot)zoltan(at)t-online(dot)hu
<mailto:szalontai(dot)zoltan(at)t-online(dot)hu> >
Sent: Thursday, April 8, 2021 01:24 PM
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Subject: procedure using CURSOR to insert is extremely slow

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