plpgsql function with offset - Postgres 9.1

Patrick B wrote:

I am running a background task on my DB, which will copy data from tableA to tableB. For
that, I'm writing a PL/PGSQL function which basically needs to do the following:

1. Select the data from tableA
2. The limit will be put when calling the function
3. insert the selected data on Step 1 onto new table

Question:

* When I stop it and start it again, how can the query "know" that it has already
processed some rows so it won't do it twice on the same rows? If it stopped on row number
100, I need it to continue on row number 101, for example.

* How can I ask the function to return the number of processed rows?

I can add a column on TableB if needed, but not on tableA.

This is what I've done so far:

CREATE or REPLACE FUNCTION data_copy(rows integer)
RETURNS SETOF bigint AS $$

CREATE or REPLACE FUNCTION data_copy(p_limit integer, p_offset integer)
RETURNS integer;

declare
row record;
offset_num integer;

num_rows integer := 0;

BEGIN
FOR row IN EXECUTE '
SELECT
id,
path,
name,
name_last,
created_at
FROM
tablea
WHERE
ready = true
ORDER BY 1 LIMIT ' || rows || ' OFFSET ' || rows || ''

'... LIMIT ' || p_limit || ' OFFSET ' || p_offset

LOOP

num_rows := num_rows + 1;

INSERT INTO tableB (id,path,name,name_last,created_at)
VALUES (row.id,row.path,row.name,row.name_last,row.created_at);

END LOOP;

RETURN num_rows;

END
$$ language 'plpgsql';

There are two problems with this approach:

1. It will do the wrong thing if rows are added or deleted in "tablea" while
you process it.

2. Queries with hight OFFSET values have bad performance.

The solution is to avoid OFFSET and to use "keyset pagination":
http://use-the-index-luke.com/no-offset

Yours,
Laurenz Albe

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2017-05-29 19:27 GMT+12:00 Albe Laurenz <laurenz.albe@wien.gv.at>:

Patrick B wrote:

I am running a background task on my DB, which will copy data from

tableA to tableB. For

that, I'm writing a PL/PGSQL function which basically needs to do the

following:

1. Select the data from tableA
2. The limit will be put when calling the function
3. insert the selected data on Step 1 onto new table

Question:

* When I stop it and start it again, how can the query "know" that

it has already

processed some rows so it won't do it twice on the same rows? If it

stopped on row number

100, I need it to continue on row number 101, for example.

* How can I ask the function to return the number of processed rows?

I can add a column on TableB if needed, but not on tableA.

This is what I've done so far:

CREATE or REPLACE FUNCTION data_copy(rows integer)
RETURNS SETOF bigint AS $$

CREATE or REPLACE FUNCTION data_copy(p_limit integer, p_offset integer)
RETURNS integer;

declare
row record;
offset_num integer;

num_rows integer := 0;

BEGIN
FOR row IN EXECUTE '
SELECT
id,
path,
name,
name_last,
created_at
FROM
tablea
WHERE
ready = true
ORDER BY 1 LIMIT ' || rows || ' OFFSET ' ||

rows || ''

'... LIMIT ' || p_limit || ' OFFSET ' || p_offset

LOOP

num_rows := num_rows + 1;

INSERT INTO tableB (id,path,name,name_last,created_at)
VALUES (row.id,row.path,row.name,row.

name_last,row.created_at);

END LOOP;

RETURN num_rows;

END
$$ language 'plpgsql';

There are two problems with this approach:

1. It will do the wrong thing if rows are added or deleted in "tablea"
while
you process it.

There will be actually records being inserted in tablea while processing
the migration.... Any ideas here?

I can add another column in tablea, like example: row_migrated boolean -->
if that helps

2. Queries with hight OFFSET values have bad performance.

No problem. The plan is to perform 2k rows at once, which is not much.

The solution is to avoid OFFSET and to use "keyset pagination":
http://use-the-index-luke.com/no-offset

Thanks
Patrick

On Thu, Jun 15, 2017 at 3:19 PM, Patrick B <patrickbakerbr@gmail.com> wrote:

2017-05-29 19:27 GMT+12:00 Albe Laurenz <laurenz.albe@wien.gv.at>:

Patrick B wrote:

I am running a background task on my DB, which will copy data from

tableA to tableB. For

that, I'm writing a PL/PGSQL function which basically needs to do the

following:

​
/messages/by-id/CY1PR18MB0490632A9A73E64973F66D6BAFCE0@CY1PR18MB0490.namprd18.prod.outlook.com

David J​

2017-06-16 10:35 GMT+12:00 David G. Johnston <david.g.johnston@gmail.com>:

On Thu, Jun 15, 2017 at 3:19 PM, Patrick B <patrickbakerbr@gmail.com>
wrote:

2017-05-29 19:27 GMT+12:00 Albe Laurenz <laurenz.albe@wien.gv.at>:

Patrick B wrote:

I am running a background task on my DB, which will copy data from

tableA to tableB. For

that, I'm writing a PL/PGSQL function which basically needs to do the

following:

​/messages/by-id/CY1PR18MB0490632A9A73E64973F66
D6BAFCE0%40CY1PR18MB0490.namprd18.prod.outlook.com

David J​

My problem is when using LIMIT to select the data I get none. I assume
because the first 3000 rows (i'm using ORDER BY 1) are not candidates for
the migration.

On Thu, Jun 15, 2017 at 3:49 PM, Patrick B <patrickbakerbr@gmail.com> wrote:

2017-06-16 10:35 GMT+12:00 David G. Johnston <david.g.johnston@gmail.com>:

On Thu, Jun 15, 2017 at 3:19 PM, Patrick B <patrickbakerbr@gmail.com>
wrote:

2017-05-29 19:27 GMT+12:00 Albe Laurenz <laurenz.albe@wien.gv.at>:

Patrick B wrote:

I am running a background task on my DB, which will copy data from

tableA to tableB. For

that, I'm writing a PL/PGSQL function which basically needs to do the

following:

​/messages/by-id/CY1PR18MB0490632A9A73
E64973F66D6BAFCE0%40CY1PR18MB0490.namprd18.prod.outlook.com

My problem is when using LIMIT to select the data I get none. I assume

because the first 3000 rows (i'm using ORDER BY 1) are not candidates for
the migration.

Your where clause should only allow candidates to be selected. If it does
then your theory is wrong.

David J.​

On 2017-06-16 10:19:45 +1200, Patrick B wrote:

2017-05-29 19:27 GMT+12:00 Albe Laurenz <laurenz.albe@wien.gv.at>:
Patrick B wrote:

I am running a background task on my DB, which will copy data from tableA

to tableB. For

that, I'm writing a PL/PGSQL function which basically needs to do the

following:

1.    Select the data from tableA
2.    The limit will be put when calling the function
3.    insert the selected data on Step 1 onto new table

[...]

               FOR row IN EXECUTE '
                           SELECT
                                   id,
                                   path,
                                   name,
                                   name_last,
                                   created_at
                           FROM
                                   tablea
                           WHERE
                                   ready = true
                           ORDER BY 1 LIMIT ' || rows || ' OFFSET ' ||

rows || ''

'... LIMIT ' || p_limit || ' OFFSET ' || p_offset

               LOOP

num_rows := num_rows + 1;

               INSERT INTO tableB (id,path,name,name_last,created_at)
               VALUES (row.id,row.path,row.name,row.

name_last,row.created_at);

               END LOOP;

[...]

There are two problems with this approach:

1. It will do the wrong thing if rows are added or deleted in "tablea"
while
   you process it.

There will be actually records being inserted in tablea while processing the
migration.... Any ideas here?

Is id monotonically increasing? You might be able to use that, as Albe
suggests:

The solution is to avoid OFFSET and to use "keyset pagination":
http://use-the-index-luke.com/no-offset

But it works only if rows cannot become ready after their id range has
already been processed. Otherwise you will miss them.

I can add another column in tablea, like example: row_migrated boolean --> if
that helps

Yes that's probably the best way. Instead of using an additional column
you could also make ready tristate: New -> ready_for_migration -> migrated.

2. Queries with hight OFFSET values have bad performance.

No problem. The plan is to perform 2k rows at once, which is not much.

Are rows deleted from tablea after they are migrated? Otherwise you will
have a problem:

select ... limit 2000 offset 1234000

will have to retrieve 1236000 rows and then discard 1234000 of them.

hp

--
_ | Peter J. Holzer | we build much bigger, better disasters now
|_|_) | | because we have much more sophisticated
| | | hjp@hjp.at | management tools.
__/ | http://www.hjp.at/ | -- Ross Anderson <https://www.edge.org/&gt;