Postgres window functions are an extremely useful feature. They let you calculate a value for each row based on the values of other rows. For our examples, let’s say we have a table ints full of random numbers:
CREATE TABLE… Read the rest
SELECT DISTINCT ON … ORDER BY
A common problem I’ve seen that many people are unfamiliar with how to solve is obtaining all of the rows that are the maximum of one value grouped by another value. For example, you may have a pets table with … Read the rest
The Missing Postgres Scan: The Loose Index Scan
Postgres has many different types of scans builtin. The list includes sequential scans, index scans, and bitmap scans. One useful scan type Postgres does not have builtin that other databases do is the loose index scan. Both MySQL … Read the rest
Postgres Lateral Joins
Personally, lateral joins are one of my favorite Postgres features. They are simple, while at the same time they let you write queries that would be nearly impossible to write otherwise. I find it surprising lateral joins were only introduced … Read the rest
Understanding Postgres Recursive CTEs
Unlike ordinary CTEs, which can be thought of as syntactic sugar, recursive CTEs make SQL a more powerful language. As the name suggests a recursive CTE makes it possible to express recursion in SQL. The ability to express recursion … Read the rest
Postgres CTEs
CTEs, also know as Common Table Expressions, are a way of refactoring a query. They let you give the result of a subquery a name and reuse the result multiple times. The general syntax of a CTE looks like the … Read the rest
Postgres Heap Only Tuples
Heap only tuples, otherwise known as HOT, is an optimization Postgres uses to reduce the amount of I/O necessary for updates. Due to MVCC, an update in Postgres consists of finding the row being updated, and inserting a new … Read the rest
Postgres Vacuum Full
One detail to note about the Postgres vacuum is that it doesn’t not return the space it frees back to the OS. Instead, it only marks the space as free, so it can later be used new rows. That means … Read the rest
Transaction Id Wraparound in Postgres
Transaction id wraparound has been the cause of numerous outages. Both Sentry and Joyent have blog posts detailing day long outages caused by transaction id wraparound. There are also many other companies that have ran into transaction id wraparound, but … Read the rest
The Postgres Vacuum
As mentioned in my post on MVCC, when a row is deleted from Postgres, the physical row is not actually deleted. All that really happens is the physical row is marked as deleted. That way, queries that started before … Read the rest