Fwd: Unexpected Multiple Records from Randomized Query
Hello. I encountered a problem while setting up a test environment to learn
the use of the random() function.
The query is as follows:
CREATE TABLE users (
id SERIAL PRIMARY KEY,
name VARCHAR(100),
email VARCHAR(100),
signup_date DATE
);
INSERT INTO users (name, email, signup_date)
SELECT
'User ' || i,
'user' || i || '@example.com',
NOW() - (random() * (365 * 5) || ' days')::interval
FROM generate_series(1, 1000000) AS s(i);
explain analyze
SELECT * FROM users WHERE name = 'User '||trunc(random()*100) ;
I expected the result to return one record. However, in some cases, the
result comes back with 2 or 3 records. What am I doing wrong?
I'm not sure if this is a conflict issue between the random() and trunc()
functions, or if I have set up the test case incorrectly.
I am using PostgreSQL 15.4 version on Ubuntu 22.04 in a container
environment. PostgreSQL was built by compiling the Source code.
Thank you for your time and consideration.
Best regards,
Myoungjun Kim
Import Notes
Reply to msg id not found: CAOKhjoz9_Fsf+NdbL08te_nWi_-hE2tycAmO7Yz4X0zZAtC2tA@mail.gmail.comReference msg id not found: CAOKhjoz9_Fsf+NdbL08te_nWi_-hE2tycAmO7Yz4X0zZAtC2tA@mail.gmail.com
=?UTF-8?B?6rmA66qF7KSA?= <audwns525@gmail.com> writes:
explain analyze
SELECT * FROM users WHERE name = 'User '||trunc(random()*100) ;
I expected the result to return one record. However, in some cases, the
result comes back with 2 or 3 records. What am I doing wrong?
random() is re-evaluated at each row, so it's not that surprising
if you sometimes get multiple matches. This is the same behavior
that you relied on to fill the table with not-all-the-same names.
The preferred way to avoid that is to stuff the random() call into
a CTE:
WITH x AS (SELECT random() AS r)
SELECT * FROM users, x WHERE name = 'User '||trunc(r*100) ;
or in this case better to shove the whole constant computation
into the CTE.
regards, tom lane