How to link old and new IDs when migrating data from one table to another

l-lin included in post

2018-03-29 252 words 2 minutes

When proceeding a database migration, I often had to create a temporary table that will contains the data from the old database.

However, after the migration, I still need to have links between the old IDs and the new IDs for various post migration operation.

This post is about creating the table containing the linked ids.

Imagine we have the following:

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CREATE TABLE old_table (
  id int4,
  name VARCHAR
);
INSERT INTO old_table (id, name) VALUES (3, 'foo');
INSERT INTO old_table (id, name) VALUES (4, 'bar');
INSERT INTO old_table (id, name) VALUES (6, 'foobar');
INSERT INTO old_table (id, name) VALUES (10, 'foo bar');

I want to migrate to this new table:

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CREATE TABLE new_table (
  id serial PRIMARY KEY,
  new_name VARCHAR
);

With some CTE magic, I should be able to get my temp table that contains the links:

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WITH sel AS (
  SELECT *, row_number() OVER (ORDER BY id) AS rn
  FROM old_table
  ORDER BY id
),
ins AS (
  INSERT INTO new_table (new_name)
  SELECT name
  FROM old_table
  ORDER BY id -- Optional, just to be sure
  RETURNING id
)
SELECT i.id AS new_id, sel.id AS old_id
FROM (SELECT id, row_number() OVER (ORDER BY id) AS rn FROM ins) i
JOIN sel USING (rn);

And VOILA, this SQL query will return the link between the old and new IDs.

Check this sqlfiddle.

Source: Stackoverflow