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Overview
Database administrator assessments centre on the things that are expensive to get wrong: recovery, performance and access. You are typically given a slow query with its execution plan, a backup and restore scenario, or a schema to review, and asked what you would do and why. This page covers what those exercises look like, how they are marked, and where candidates most often lose points.
Why employers assess this role
A database is usually the component an organisation cannot afford to lose, and a recovery plan that has never been tested is discovered to be a recovery plan that does not work at the worst possible moment. Employers assess practically because performance tuning and recovery judgement are difficult to fake in conversation and impossible to read from a list of database products on a CV.
What gets tested
The format
Duration
60–90 minutes
Question types
- Query tuning exercise with an execution plan
- Backup and recovery scenario
- Schema and index review
- Written reasoning on availability and maintenance trade-offs
Levels
Entry · Mid · Senior
What you'll be asked to do
Tune a slow query
The most common exercise. You are given a query, a plan and some table statistics, and asked to make it faster and explain what you changed and why.
- •Diagnose a query whose plan shows a full scan on a large table despite an available index
- •Rewrite a query where a function applied to a column is preventing index use
- •Explain why a query is fast for one parameter value and slow for another
Recover from a failure
A scenario is described — corruption, an accidental deletion, a lost server — and you set out the recovery. Assessors look for whether recovery objectives are stated and met.
- •Recover a table dropped in error four hours ago with transaction logs available
- •Restore to a point in time immediately before a bad deployment
- •Decide the recovery order when both the primary and its backup storage are affected
Review a schema
A schema with real problems in it — missing constraints, poor data types, an index strategy that will not hold up. You are scored on what you catch and how you prioritise fixing it.
- •Identify the missing constraints that allow invalid data into a given schema
- •Assess whether a denormalised table is justified by its read pattern
- •Propose an index strategy for a stated mix of queries and write volume
Design for availability and maintenance
How the database survives failure and how it gets maintained without downtime nobody agreed to. This section carries most of the senior-level weight.
- •Choose a replication topology to meet a stated recovery point objective
- •Plan a schema change on a large table without a long lock
- •Define what monitoring would have caught a described incident before users did
Cohesyve for candidates
Practise a Database Administrator assessment before the real one
Run the same AI job simulations companies use to evaluate applicants. You get a scored report showing where you're strong and where you're not, plus what to work on.
Sample tasks — and what strong looks like
A reporting query that used to run in two seconds now takes ninety. Nothing about the query has changed. Diagnose it.
MidWhat strong looks like: Compares the current plan against what it should be, considers data growth, stale statistics, index fragmentation and plan changes, and identifies which one the evidence supports before making a change. Weak answers add an index immediately without establishing why the previous plan stopped working.
Someone ran a delete without a where clause on a production table three hours ago. Set out the recovery.
SeniorWhat strong looks like: Stops further writes or isolates the table first, establishes exactly when the statement ran, recovers to a point immediately before it using the log chain, and plans how to reconcile legitimate changes made in the intervening three hours. Weak answers restore the last full backup and lose everything since, without mentioning it.
Given a table definition and the five most common queries against it, propose an index strategy.
MidWhat strong looks like: Chooses composite indexes with column order driven by the actual predicates, covers the queries that matter without duplicating existing indexes, and states the write cost each new index introduces. Weak answers index every column that appears in a where clause and ignore the impact on inserts.
Add a non-nullable column with a default to a table with hundreds of millions of rows on a system that cannot take extended downtime.
SeniorWhat strong looks like: Recognises the lock and rewrite risk, proposes a staged approach — add nullable, backfill in batches with monitoring, then apply the constraint — and includes a rollback at each stage. Weak answers issue a single statement and treat the lock duration as an unknown they will find out about during the change.
How to prepare
- #1
Practise reading execution plans until the expensive operator is obvious at a glance, because plan interpretation is the single most frequently assessed skill in these exercises.
- #2
Rehearse a full restore in a test environment, including point-in-time recovery, so that recovery steps come from experience rather than from documentation you half remember.
- #3
Learn to state recovery point and recovery time objectives explicitly in any recovery answer, since assessors look for those numbers and most candidates never mention them.
- #4
Work through index design against realistic query mixes and force yourself to name the write cost of every index you propose.
- #5
Practise planning a schema change on a large table with an explicit lock analysis and rollback, as this is where senior marks are concentrated.
- #6
Complete one full timed assessment end to end, because tuning exercises absorb time and the recovery section is frequently left half finished.
Common mistakes
Adding indexes to fix a slow query without first reading the execution plan.
Describing a backup strategy with no evidence that a restore has ever been tested.
Ignoring the write and storage cost of an index strategy.
Recovering to the last full backup and silently discarding hours of legitimate transactions.
Proposing a schema change on a large table without considering lock duration or blocking.
Granting broad privileges for convenience rather than scoping access to what the application needs.
How it's scored
| Criterion | What strong looks like |
|---|---|
| Performance diagnosis | The cause is identified from the plan and statistics before any change is proposed, and the improvement is verified rather than assumed. |
| Recovery competence | Recovery point and recovery time are stated, the log chain is used where available, and data loss is quantified and acknowledged rather than glossed over. |
| Schema and integrity | Constraints enforce correctness in the database itself, data types are appropriate, and any denormalisation is justified by a stated access pattern. |
| Operational safety | Changes to live systems are staged, monitored and reversible, with lock and blocking behaviour considered before execution. |
| Access control | Privileges are scoped to the minimum required, sensitive columns are handled deliberately, and administrative access is separated from application access. |
Frequently Asked Questions
Which database engine will I be tested on?
Usually the one the employer runs, and it is normally named beforehand. The reasoning about plans, indexes and recovery transfers well between engines, so understanding why a plan is expensive matters more than knowing one product's syntax for showing it.
How much SQL writing is involved?
Less than candidates expect. Most of the marks sit in reading queries and plans, proposing changes and explaining trade-offs, though you should expect to write or rewrite at least one query.
Do these assessments cover cloud managed databases?
Frequently, yes. Managed services change who performs the maintenance but not the reasoning, so questions about backups, replicas, failover and performance still come up in essentially the same form.
What is the most common reason candidates lose marks?
Acting before diagnosing. Adding an index or restoring a backup without first establishing the cause and the objectives loses points even when the outcome happens to be right.
Can I practise a scored assessment beforehand?
You can, and it is worth doing before an employer marks you. Cohesyve provides three free scored assessments with a full report, so you can see exactly which criteria your tuning and recovery answers fall short on.
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Cohesyve · Practice for candidates
Practise a Database Administrator assessment now — free.
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3
free assessments
$0
no card required
Full
scored report every run
Every
question type included
3 free assessments · No card required · Pro from $16/mo
For hiring teams
Hiring for a Database Administrator role? See how your applicants perform before you spend interview time.