CockroachDB: Fixes For Empty Column Set Statistics Error

Hey folks! Ever run into a weird error message that makes you scratch your head? We've got one that popped up in Sentry, and it's a doozy: statistics_builder.go:444: column statistics cannot be determined for empty column set (1) assertion failure. Sounds cryptic, right? Don't worry, we're gonna break it down for you. This error is happening within the CockroachDB ecosystem, specifically in the statistics_builder.go file at line 444. It's essentially an assertion failure, meaning something in the code expected a certain condition to be true, but it turned out to be false. In this case, the optimizer was trying to figure out statistics for a column (or columns), but it found an empty column set. This is a bit like asking a chef to make a dish with no ingredients – it just can't be done!

This particular error seems to be triggered during operations that involve set logic, like EXCEPT statements, and it's bubbling up through a whole chain of functions related to query planning and optimization. We're talking about the bowels of the database engine here, where it figures out the most efficient way to run your queries. When it hits this snag, it means the optimizer got confused because it encountered a situation where it needed to calculate statistics on something that didn't have any columns defined. This can happen in some edge cases, especially with complex queries involving WITH clauses or EXCEPT operations. The stack trace gives us a pretty good roadmap of where this is happening, starting from the user connection (pgwire) all the way down to the optimizer's statistics builder. So, while it looks scary, it's a sign that the database hit an unexpected state during query optimization. We'll dive deeper into what this means and how it might be resolved.

Diving Deeper: The Mechanics of the Error

Alright guys, let's get a bit more technical about this statistics_builder.go:444 error. What's really going on under the hood? When CockroachDB processes a query, it doesn't just run it blindly. Oh no, it has this super smart optimizer that tries to figure out the best way to execute your SQL. Part of this optimization process involves understanding the data itself – how many rows are there, what are the typical values in a column, are there many duplicates, and so on. This information is called statistics. The optimizer uses these statistics to make educated guesses about how different query plans will perform. For example, if it knows a column has only a few unique values, it might choose a different join strategy than if it knows the column has millions of unique values.

Now, the error we're seeing, column statistics cannot be determined for empty column set, means that at a crucial point in this process, the statistics builder encountered a situation where it was asked to collect statistics on a set of columns that, well, didn't exist. Think of it like trying to measure the height of a group of people, but you're only given an empty box. You can't measure anything! This usually happens when the query involves operations that might result in an empty set of columns downstream, like maybe an EXCEPT clause where one of the sides of the EXCEPT doesn't return any columns, or perhaps a WITH clause that generates an unexpected output. The stack trace shows this error originating deep within the pkg/sql/opt/memo package, specifically in statistics_builder.go. This package is all about building the memoization table, which is a core data structure used by the optimizer to store and reuse intermediate results and plans. When the colStat function (or one of its helpers like colStatFromChild) is called, it expects to find some column information to analyze. If it gets an empty list, it throws this assertion failure because it's a condition it's not designed to handle gracefully.

This isn't necessarily a bug in your query syntax itself, but more likely an internal hiccup within the optimizer when it's trying to make sense of a particular query structure. It's the optimizer hitting a logical inconsistency. The fact that it's wrapped in *assert.withAssertionFailure and *withstack.withStack just confirms it's an internal programming error that the system detected and is reporting. We'll explore the implications and potential fixes in the next sections.

Why Does This Happen? Potential Causes and Scenarios

So, why would the optimizer suddenly face an empty column set? It's not like you explicitly told it to do that! Usually, these kinds of errors pop up due to the combinatorial complexity of SQL query processing. The optimizer explores many different ways to execute a query, and sometimes, in certain complex scenarios, it can arrive at a state where it expects columns that aren't actually there. Let's chat about some likely culprits, guys:

1. Set Operations with Mismatched Structures: The stack trace heavily points towards set operations like EXCEPT. Imagine you have a query like SELECT a, b FROM table1 EXCEPT SELECT x FROM table2. If the SELECT x FROM table2 part somehow results in a plan that doesn't output any columns (which is weird in itself, but possible in very specific internal states), the EXCEPT operator would then try to compare it with a, b from table1. When it comes time to calculate statistics for the EXCEPT operation, it might look at the