I have the following query in postgres:

SELECT * 
FROM "bookings"
WHERE ("bookings".client_id = 50) 
ORDER BY session_time DESC 
LIMIT 20 OFFSET 0

The record in the 20th place is has an identical session_time to the 21st record.

This query returns 20 results, however if you compare the results to the whole database the query returns the 1st-19th results and the 21st, skipping over the 20th.

This query can be fixed by adding, "id" to the order:

SELECT * 
FROM "bookings" 
WHERE ("bookings".client_id = 50) 
ORDER BY session_time DESC, id 
LIMIT 20 OFFSET 0

However I was wondering how this bug occurred? How does postgres order identical filed when using offsets and limits? Is it random? Is it a bug with postgres?

sql postgresql sql-order-by limit offset
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