Re: efficiently migrating 'old' data from one table to another

On 01/12/2017 12:06 PM, Jonathan Vanasco wrote:
> I'm just wondering if there's a more efficient way of handling a certain periodic data migration.
>
> We have a pair of tables with this structure:
>
> table_a__live
> column_1 INT
> column_2 INT
> record_timestamp TIMESTAMP
>
> table_a__archive
> column_1 INT
> column_2 INT
> record_timestamp TIMESTAMP
>
> periodically, we must migrate items that are 'stale' from `table_a__live ` to `table_a__archive`. The entries are copied over to the archive, then deleted.
>
> The staleness is calculated based on age-- so we need to use INTERVAL. the "live" table can have anywhere from 100k to 20MM records.
>
> the primary key on `table_a__live` is a composite of column_1 & column_2,
>
> In order to minimize scanning the table, we opted to hint migrations with a dedicated column:
>
> ALTER TABLE table_a__live ADD is_migrate BOOLEAN DEFAULT NULL;
> CREATE INDEX idx_table_a__live_migrate ON table_a__live(is_migrate) WHERE is_migrate IS NOT NULL;
>
> so our migration is then based on that `is_migrate` column:
>
> BEGIN;
> UPDATE table_a__live SET is_migrate = TRUE WHERE record_timestamp < transaction_timestamp() AT TIME ZONE 'UTC' - INTERVAL '1 month';
> INSERT INTO table_a__archive (column_1, column_2, record_timestamp) SELECT column_1, column_2, record_timestamp FROM table_a__live WHERE is_migrate IS TRUE;
> DELETE FROM table_a__live WHERE is_migrate IS TRUE;
> COMMIT;
>
> The inserts & deletes are blazing fast, but the UPDATE is a bit slow from postgres re-writing all the rows.

Maybe I am missing something, but why do the UPDATE?

Why not?:

BEGIN;

INSERT INTO
table_a__archive (column_1, column_2, record_timestamp)
SELECT
column_1, column_2, record_timestamp
FROM
table_a__live
WHERE
record_timestamp < transaction_timestamp() AT TIME ZONE 'UTC' - INTERVAL '1 month';

DELETE FROM
table_a__live
WHERE
record_timestamp < transaction_timestamp() AT TIME ZONE 'UTC' - INTERVAL '1 month';
COMMIT;

With an index on record_timestamp.

>
> can anyone suggest a better approach?
>
> I considered copying everything to a tmp table then inserting/deleting based on that table -- but there's a lot of disk-io on that approach too.
>
>
> fwiw we're on postgres9.6.1
>

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com