Re: Creating 1 million users in PostgreSQL

Hans-Jürgen Schönig wrote a great post “Creating 1 million users in PostgreSQL”. I just love such things! Well done! Thanks.

But since there is no possibility to leave comments under original post (BTW why?), I’ll create my own post.

1. When testing such tricks always create temp database. Because I accidentally used… no, no, not the production one.. but anyway. 🙂
   UPD:

   Since when temp database matters when creating cluster-wide role?
   © Ervin Weber

   So, yes. There is no need in test database. My bad! 🙂

2. Hans-Jürgen used shell, psql and \gexec combination. However, we may do all these steps using psql only

To make a long story short, this is my variant of creating 1 million users in PostgreSQL:

Test=# CREATE DATABASE test_role_db;
CREATE DATABASE
Time: 7947,657 ms

Test=# \c test_role_db
You are now connected to database "test_role_db".

test_role_db=# \timing
Timing is on.

test_role_db=# DO $$
test_role_db$# BEGIN
test_role_db$#   FOR i IN 1..1000000 LOOP
test_role_db$#     EXECUTE 'CREATE ROLE xy' || i;
test_role_db$#   END LOOP;
test_role_db$# END; $$;
DO
Time: 81267,482 ms

On my test environment generating 1 million users in a single transaction takes around 1 minute 21 seconds. This is a little bit longer than Hans-Jürgen’s timing. Suppose this is due to the hardware difference.

Serial-Identity mix

Serial-identity

As you probably know PostgreSQL 10 introduced IDENTITY columns. They are pretty much like serial columns from the first sight. To learn about differences one may refer to the perfect blog post by Peter Eisentraut.

So I was wondering what if I mix some serial with some identity. So I executed such simple query:

CREATE TABLE foo(
id SERIAL PRIMARY KEY GENERATED BY DEFAULT AS IDENTITY
);

Of course, I got an error:

ERROR: both default and identity specified for column "id" of table "foo"
SQL state: 42601

“SQL state: 42601” stands for a syntax error. But the interesting thing is that SERIAL, as you know, converted to CREATE SEQUENCE and SET DEFAULT.

So now I know what exactly the reason of error. You cannot have both DEFAULT and IDENTITY for a column.

Creating titles from text (SQL)

Task: get title (or excerpt) from given text (body)
Solution:

SELECT
left(body, 29) ||
COALESCE( substring( substr( body, 30 ) from '(.+?)(\,| )' ), '' ) as title,


Details: Take the first 29 characters, concatenate them with substring started from the 30-th character till the first occurrence of comma or space. Or concatenate with empty string, if pattern doesn’t match (COALESCE).

MERGE in PostgreSQL

Found cool trick how today implement Orable MERGE in PostgreSQL:

Oracle statement:

 

2. MERGE INTO acme_obj_value d
3. USING ( SELECT object_id
4.         FROM   acme_state_tmp
5.       ) s
6. ON (d.object_id = s.object_id)
7.   WHEN matched THEN
8.     UPDATE SET d.date_value = LEAST(l_dt, d.date_value)
9.   WHEN NOT matched THEN
10.     INSERT (d.id, d.object_id, d.date_value)
11.     VALUES (acme_param_sequence.NEXTVAL, s.object_id, l_dt)

PostgreSQL statement:

 

2. WITH s AS (
3.      SELECT object_id
4.      FROM   acme_state_tmp
5. ),
6. upd AS (
7.      UPDATE acme_obj_value
8.      SET    date_value = LEAST(l_dt, d.date_value)
9.      FROM   s
10.      WHERE  acme_obj_value.object_id = s.object_id
11.      RETURNING acme_obj_value.object_id
12. )
13. INSERT INTO acme_obj_value(id, object_id,  date_value)
14. SELECT NEXTVAL(‘acme_param_sequence’), s.object_id, l_dt
15. FROM   s
16. WHERE  s.object_id NOT IN (SELECT object_id FROM upd)

 

Delphi: Warnigs → Errors

Want to force your dev team to address Compiler Warnings rather than ignoring them?

Elevate Warnings to Errors:

1) Go to Project Options
2) Under Delphi Compiler, select Hints and Warnings
3) Expand the Output warnings item
4) For each important Warning you wish for your developers to address rather than ignore, select the drop-down list and select Error.

Enjoy!

Thanks Simon Stuart for hint!