Add column with default in postgresql without table level lock

Have such a problem - table with over 20mln rows.

When i add new column with default - postgresql lock table for over 40 minutes so my application stop working for this time.

So instead of

ALTER TABLE "test" ADD COLUMN "field" boolean DEFAULT True NOT NULL;

I do

ALTER TABLE "test" ADD COLUMN "field" boolean NULL;
ALTER TABLE "test" ALTER COLUMN "field" SET DEFAULT true;

after which every new row will be with true by default, so now i need to update 20mln current rows.
I update them in batches:

WITH cte AS (
SELECT id as pk
FROM "test"
WHERE  "field" is null
LIMIT  10000
)
UPDATE "test" table_
SET "field" = true
FROM   cte
WHERE  table_.id = cte.pk

after which i do

ALTER TABLE "test" ALTER COLUMN "field" SET NOT NULL;

And everything is allwright but step when i update rows is so much slow. Can you give my some advice about improving speed of update?

Currently it update 10000 in about 2 minutes.
I try decreasing size to 1000 - it was better (3.5 minutes for 10000) but it still to slow.
I also try create index before update, but it doesnt give better results (as i understand it will give better result but when huge path of table will be updated).

This problem is solved in PostgreSQL 11, which was released on October 18, 2018.

There is a more detailed explanation in this blog post, but the short version is that in Version 11 the table doesn't have to be rewritten to add a default value to a column. So adding a column with a default should be pretty much instant.

StackExchange Database Administrators Q#211221, answer score: 15