C Developer DBMS Concepts Interview Guide

At Read Committed, each statement receives a new snapshot, so later statements can see commits made in between. Higher isolation changes the guarantees and may require retrying serialization failures. Choose isolation from the business invariant, not from an assumption that every transaction freezes all reads.

Check your understanding: Interleave two sessions and document the snapshots each statement sees.

Common trap: Treating transaction atomicity and isolation as identical.

Concept reference: PostgreSQL: transaction isolation

An index is an option, not a command. The planner considers selectivity, statistics, table size and access cost. Read estimated and actual rows, filters and scan types before changing indexes. EXPLAIN ANALYZE executes the statement, so use safe data and transaction handling for writes.

Check your understanding: Compare estimated and actual row counts on representative data.

Common trap: Forcing an index without measuring the full query.

Concept reference: PostgreSQL: reading EXPLAIN plans

Check the grain of each table and whether the join creates several rows for one order. Aggregate at the intended business grain before joining, or use the correct relationship and key. Reconcile totals using a tiny fixture containing multiple child rows and an unmatched parent.

Check your understanding: Use one order with two child records and verify the total remains correct.

Common trap: Using DISTINCT as a universal repair for a wrong join.

Concept reference: PostgreSQL: reading EXPLAIN plans

The object’s lifetime ends when execution leaves its scope. The pointer does not extend that lifetime, even if the memory still appears readable. Return a value, accept caller-owned storage, or allocate an object with a documented ownership and release contract.

Check your understanding: Identify the lifetime of each object in a small pointer-returning function.

Common trap: Equating a non-null pointer with a live object.

Concept reference: WG14: C11 committee draft N1570

Validate the intended limits and ensure multiplication cannot overflow the size type before allocation. Handle allocation failure and keep later indexing within the allocated object. An overflow can create a much smaller allocation than the caller expects even when both inputs look individually plausible.

Check your understanding: Test zero, maximum valid size and a product exceeding the size-type range.

Common trap: Checking only the allocation return value after overflow already occurred.

Concept reference: WG14: C11 committee draft N1570

Signed overflow is not a portable wraparound operation in C. Optimizers may rely on the language’s defined-behavior rules when transforming code. Check bounds before arithmetic or use a deliberately chosen representation and checked operation. Specify the target standard and implementation assumptions.

Check your understanding: Exercise boundary values with suitable compiler diagnostics and sanitizers.

Common trap: Depending on one machine’s observed signed wraparound.

Concept reference: WG14: C11 committee draft N1570

First, identify technical constraints and define measurable service objectives. Next, trace resource ownership and state transitions across the critical path. Contrast architectural trade-offs across write cost, read latency, consistency and operational complexity, explicitly mitigate the risk of keyboard and assistive-technology users cannot complete the critical action, and confirm system stability using an accessibility audit, keyboard trace and corrected acceptance test.

Common trap: Reaching for a specific library or framework before defining constraints, failure envelopes, and automated verification criteria.

Prioritise the failure mode exhibiting the highest user blast radius and lowest observability. Formulate an explicit containment boundary, implement idempotent retries with jitter, and establish an automated rollback threshold. Verify resilience through a server-authoritative timestamp trace and boundary property tests.

Common trap: Relying on passive monitoring dashboards without defining explicit error-budget alerts, rollback triggers, and verified recovery procedures.

Structure the response using Context-Decision-Tradeoff-Result: articulate the business and technical constraints, compare viable alternatives, explain the implementation (trace resource ownership and state transitions across the critical path), and document the accepted trade-off. Provide concrete proof: an explain plan, concurrency test and recovery verification.

Common trap: Speaking only in high-level abstractions or team accomplishments without detailing your direct implementation decisions, trade-offs, and measured results.

Formulate a falsifiable hypothesis from observable telemetry before altering configurations. Then inspect allocation lifetime, synchronization, timing traces and boundary conditions. Isolate the defect to the smallest reproducible boundary, validate root cause with evidence, and confirm full resolution using a compatibility contract, expand-and-contract rollout and rollback rehearsal.

Common trap: Applying speculative fixes or restarting services blindly without establishing an observable signal connected to a falsifiable hypothesis.

Produce a sanitizer or ownership trace with a deterministic stress test. Document baseline assumptions, technical mechanism (trace resource ownership and state transitions across the critical path), rejected alternatives, bounded failure envelopes, and deterministic pass criteria. Supply reproducible verification via a before-and-after latency profile plus an explicit rollback threshold.

Common trap: Presenting architecture diagrams or slides lacking automated unit/integration tests, observable metrics, or automated rollback configurations.