Question: What is the difference between P50 and P90 latency in database performance?

Answer

P50 and P90 are statistical terms used to understand the performance of a system, such as a database, under load. They are percentiles that help interpret the distribution of response times.

P50 Latency (also known as Median Latency): This value represents the point below which 50% of the observations fall. In other words, 50% of your queries will get a response in less time than the P50 latency.

P90 Latency: P90 latency is the point where 90% of the measurements fall below that value. It means 90% of your queries will get a response in less than the P90 latency.

The main reason to measure P50, P90, or even P99 latencies instead of average latency is because they give a much better understanding of outliers. Average latency can be heavily influenced by a few extremely slow requests, whereas percentiles like P90 give a more realistic picture of what most users are experiencing.

Here's a simple Python example showing how you might calculate P50 and P90 latencies from a list of latency data:

import numpy as np

# Assume this list contains latency data in milliseconds
latencies = [13, 18, 14, 35, 24, 18, 23, 45, 32, 28, 19, 26, 21, 29, 34]

p50_latency = np.percentile(latencies, 50)
p90_latency = np.percentile(latencies, 90)

print(f'P50 Latency: {p50_latency} ms')
print(f'P90 Latency: {p90_latency} ms')

In this example, if the output is P50 Latency: 24 ms and P90 Latency: 35 ms, it means that 50% of your requests are served within 24 ms, and 90% of your requests are served within 35 ms.

Analyzing P50 and P90 latencies can help you identify if a significant proportion of your queries are slower than an acceptable threshold. It allows you to optimize your database for better overall performance.