Great article, I always like to structure my queries with CTEs and I was (wrongly) assuming it all gets inlined at the end.
Sometimes it also gets complicated since these intermediate results can't be easily seen in a SQL editor. I was working on a UI to parse CTE queries and then execute them step by step to show the results of all the CTEs for easier understanding of the query (as part of this project https://github.com/sqg-dev/sqg/)
CTE stands for Common Table Expressions in SQL. They are temporary result sets defined within a single query using the WITH clause, acting like named subqueries to improve readability and structure.
I've always thought of CTEs as a code organisation tool, not an optimisation tool. The fact the some rdbms treats them as an optimisation fence was a bug, not a feature.
> Recursive CTEs use an iterative working-table mechanism. Despite the name, they aren't truly recursive. PostgreSQL doesn't "call itself" by creating a nested stack of unfinished queries.
If you want something that is more like actual recursion (I.e., depth-first), Oracle has CONNECT BY which does not require the same kind of tracking. It also comes with extra features to help with cycle detection, stack depth reflection, etc.
If your problem is aligned with the DFS model, the oracle technique can run circles around recursive CTEs. Anything with a deep hierarchy and early termination conditions is a compelling candidate.
Great article, I always like to structure my queries with CTEs and I was (wrongly) assuming it all gets inlined at the end. Sometimes it also gets complicated since these intermediate results can't be easily seen in a SQL editor. I was working on a UI to parse CTE queries and then execute them step by step to show the results of all the CTEs for easier understanding of the query (as part of this project https://github.com/sqg-dev/sqg/)
Use the term, never define the term, classic.
CTE stands for Common Table Expressions in SQL. They are temporary result sets defined within a single query using the WITH clause, acting like named subqueries to improve readability and structure.
OP here, damn - that's a very good point. Can't believe I missed it.
From the headline, I thought it might be about sports-related concussions!
I was morbidly curious what a "good CTE" could possibly be...
I wrangle databases by day, and do martial arts of an evening. Two arenas where CTEs can cause significant headaches!
I've always thought of CTEs as a code organisation tool, not an optimisation tool. The fact the some rdbms treats them as an optimisation fence was a bug, not a feature.
Improved readability is definitely the primary benefit.
Great post - thanks. I think the columns in the index you suggested in the pre-pg12 section are in the wrong order (that index would get used)
Thanks - I will recheck later today.
> Recursive CTEs use an iterative working-table mechanism. Despite the name, they aren't truly recursive. PostgreSQL doesn't "call itself" by creating a nested stack of unfinished queries.
If you want something that is more like actual recursion (I.e., depth-first), Oracle has CONNECT BY which does not require the same kind of tracking. It also comes with extra features to help with cycle detection, stack depth reflection, etc.
If your problem is aligned with the DFS model, the oracle technique can run circles around recursive CTEs. Anything with a deep hierarchy and early termination conditions is a compelling candidate.