Question 1

FOR UPDATE vs FOR UPDATE NOWAIT vs FOR UPDATE SKIP LOCKED — which to use when?

Accepted Answer

**Target Audience:** Developers who know basic transactions and want to understand locking reads from zero; candidates who need a crisp "which one when" answer.
**Difficulty:** Medium
**Engines:** PostgreSQL 16.15 (two real `psql` sessions for every locking behaviour, the verifier for the examples), SQLite via sql.js 1.10.2 for the runnable query and to confirm it has no `FOR UPDATE`. MySQL and SQL Server facts are cited, not run.

> **How to read this doc:** Every idea is explained in plain lan

Question 2

SKIP LOCKED for job queues — how do you build a concurrent queue without race conditions?

Accepted Answer

**Target Audience:** Backend developers who want a reliable job queue on PostgreSQL without a broker, and candidates asked to design one in an interview.
**Difficulty:** Hard
**Engines:** PostgreSQL 16.15 (two real `psql` sessions for every race, the verifier for the claim and plan) and SQLite via sql.js 1.10.2 for the runnable claim and reaper. MySQL facts are cited, not run.

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so y

Question 3

Recursive CTE with cycle detection — how do you handle circular manager references and produce a search order?

Accepted Answer

**Target Audience:** Developers who can write a recursive CTE and now need it to survive bad data; candidates asked the "what if the hierarchy has a loop?" follow-up.
**Difficulty:** Hard
**Engines:** SQLite 3.45.0 (sql.js 1.10.2, the engine the runnable examples on this page use) and PostgreSQL 16.15, both run for this page. The `CYCLE` and `SEARCH` clauses are PostgreSQL 14 and later.

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview t

SQLDECODED.

Medium

FOR UPDATE vs FOR UPDATE NOWAIT vs FOR UPDATE SKIP LOCKED — which to use when?

Hard

SKIP LOCKED for job queues — how do you build a concurrent queue without race conditions?

Recursive CTE with cycle detection — how do you handle circular manager references and produce a search order?