PostgreSQL - Only Update If Greater Than

A quick note on a very convenient PostgreSQL function that I learned today - GREATEST - which can be used when you want a database column updated only if the incoming value is greater (more recent) than the existing value in the column.

I’m coming from a MS SQL Server background, so I wasn’t familiar with GREATEST / LEAST, which apparently are available in most other database engines, though they are not part of the SQL standard.1 From the PostgreSQL docs:

The GREATEST and LEAST functions select the largest or smallest value from a list of any number of expressions.

While this is helpful to know, what I found even more useful was that you can use GREATEST to compare the current value of a column with an incoming value. Because the greater of the two is returned, this comparison can be used to conditionally update a column. That is, GREATEST functions almost like an if statement - update this column only if the provided value is greater than the current value:

-- comparing numeric values

UPDATE my_table
SET max_age = GREATEST(max_age, :new_value)
WHERE id = :id

-- max_age is changed only if :new_value is larger

As you’d expect, GREATEST can be used to compare numeric values, but I was very surprised to learn that it also works for comparing timestamps (which you wouldn’t necessarily realize just by reading the documentation):

-- comparing date/times!

UPDATE my_table
SET last_seen_at = GREATEST(last_seen_at, :new_value)
WHERE id = :id

-- last_seen_at is changed only if :new_value is more recent

This was my use case; I had a table with columns tracking a value’s total number of occurrences and the date/time of the latest. The logs being parsed were not in chronological order and came from various sources. Using GREATEST simplified the logic needed during the update process - I could use a single query to increment the count every time, but only change the “most recent” date column when a newer timestamp appeared:

UPDATE my_table
SET total_count = total_count + 1,
last_seen_at = GREATEST(last_seen_at, :new_value)
WHERE id = :id

Credit where it’s due - this post on EBD goes into a lot of detail, with examples, about how you can use GREATEST and LEAST - I found it practical and informative. Apparently these functions even work for strings - in effect, providing alphabetical comparison (though I don’t know if that’s necessarily the “right tool for the job”). Just another crazy thing that you can do with PostgreSQL.

  1. Here’s the GREATEST / LEAST documentation for MySQL, MariaDB, Oracle, and Db2 - with credit to this StackExchange post for those links.