# Average Search Endpoint Latency

> One endpoint. Average speed.

Canonical URL: <https://datadriven.io/problems/average_search_endpoint_latency>

Domain: SQL · Difficulty: easy · Seniority: L3

## Problem

Autocomplete feels sluggish and the frontend team suspects the search backend is the bottleneck. What is the average latency for API calls hitting the '/api/v1/search' endpoint?

## Worked solution and explanation

### Why this problem exists in real interviews

Built around `api_calls`, this problem requires grouped aggregation. Interviewers watch whether candidates handle the relationship between `endpoint`, `method`, `status` correctly under grouping pressure.

---

### Break down the requirements

#### Step 1: Group by `endpoint`

`GROUP BY` at the correct grain produces one row per group.

#### Step 2: Compute `AVG(latency)`

The AVG function computes the avg per group.

#### Step 3: Order by the metric

Sort by `avg_latency` desc for readability.

---

### The solution

**Group-aggregate for average search endpoint latency**

```sql
SELECT
    endpoint,
    AVG(latency) AS avg_latency
FROM api_calls
GROUP BY endpoint
ORDER BY avg_latency DESC
```

> **Cost Analysis**
>
> The main table has 150M rows (38 GB). Partitioned on `call_time`, so queries filtering on that column skip most partitions. The GROUP BY reduces the row count early, keeping downstream operations cheap.

> **Interviewers Watch For**
>
> Strong candidates state the correct `GROUP BY` grain before writing any SQL, showing they think about the output shape first.

> **Common Pitfall**
>
> Selecting a non-aggregated column without including it in `GROUP BY` is the most common error. Some engines reject it; others silently return arbitrary values.

---

## Common follow-up questions

- The `err_msg` column in `api_calls` has roughly 95% NULLs. How does your query handle those rows, and would the result change if NULLs were replaced with zeros? _(Tests whether the candidate understands how NULLs propagate through aggregation functions and whether their WHERE/JOIN conditions implicitly filter them out.)_
- Your GROUP BY aggregates `call_id` from `api_calls`. If two groups have the same aggregate value, how is the output ordered, and is that deterministic? _(Tests awareness that ORDER BY on a non-unique value produces non-deterministic row order without a tiebreaker.)_
- `call_id` in `api_calls` has ~150M distinct values. What index strategy keeps your query from doing a full table scan? _(Tests whether the candidate can design indexes for high-cardinality columns and understands selectivity.)_
- Could you express this same logic as a single query without CTEs or subqueries? What readability trade-off does that introduce? _(Tests whether the candidate can flatten nested logic and understands when decomposition aids maintainability.)_

## Related

- [All practice problems](https://datadriven.io/problems)
- [Mock interview mode](https://datadriven.io/interview/average_search_endpoint_latency)
- [SQL Interview Questions](https://datadriven.io/sql-interview-questions)
- [Data Engineering Interview Prep Guide](https://datadriven.io/data-engineer-interview-prep)
- [Daily Challenge](https://datadriven.io/daily)

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