Sampling in PostgreSQL with BERNOULLI Function
Tags: postgres, database, sql, datascience
Published: 2023-07-18 16:09:02

## Introduction:


As the volume of data continues to grow exponentially, efficiently analyzing large datasets has become a critical challenge for data engineers and analysts.In a current project which uses a postgreSQL database, I recently encountered a data sampling issue, but not in my role as a data analyst, but rather as a developer trying to create an algorithm that uses a portion of a sizable dataset per iteration.

 In the world of relational databases, PostgreSQL stands out as a powerful and flexible option. One of its lesser-known yet immensely useful features is sampling, which allows users to work with representative subsets of data for faster analysis. In this blog post, we will explore the BERNOULLI sampling function in PostgreSQL, its benefits, and how it can be utilized to enhance query performance.


## What is Sampling in PostgreSQL?

In the context of databases, sampling is the process of selecting a random selection of data for analysis from a larger dataset. It is especially helpful when working with large tables, where it may be time-consuming and resource-intensive to execute complicated queries on the full dataset. The BERNOULLI sampling function in PostgreSQL enables users to choose random rows from a table based on a given probability.

## Understanding the BERNOULLI Function:

The BERNOULLI sampling function in PostgreSQL is a powerful tool to perform random sampling on a table. Its syntax is simple:

```sql
SELECT * FROM table_name TABLESAMPLE BERNOULLI(sample_percentage);
```
Here, "table_name" represents the name of the table from which you want to sample, and "sample_percentage" is the percentage of the table's rows you wish to include in the sample. The sample_percentage value is a floating-point number between 0 and 100, representing the percentage of the total rows to be included in the sample.

## Benefits of Using BERNOULLI Sampling:

1. Improved Query Performance:  A smaller, more representative sample of the data can be used to run queries much faster. When working with huge tables, where conventional searches could be resource-intensive, this is very useful.

2. Efficient Data Exploration: Without having to examine the complete dataset, sampling enables data analysts to quickly grasp the properties and trends of the dataset. For data exploration and hypothesis testing, it's a useful step.

3. Reduced Overhead: When dealing with real-time or near-real-time data analysis, generating a sample can provide quick insights while reducing computational overhead.

4. Statistical Validity: The BERNOULLI sampling technique guarantees that each table row has an equal probability of being included in the sample, producing findings that are statistically valid.

Utilizing BERNOULLI Sampling in Practice:

Let's walk through a practical example to illustrate how BERNOULLI sampling can be used in PostgreSQL.

Suppose we have a massive sales table with millions of records and want to perform an exploratory analysis on a random subset of the data:

```sql
-- Select a random 10% sample from the sales table
SELECT * FROM sales TABLESAMPLE BERNOULLI(10);
```

By running this query, we can efficiently retrieve a representative 10% sample of the sales data without loading the entire table, saving time and resources. 

For my project, the requirement was slightly different, I must sample the full dataset rather than simply a subset of it based on a particular criterion. For each iteration, 3 random samples from the entire dataset should be returned

```sql
-- Select entire dataset from the garments table
 select * from  garments  tablesample BERNOULLI(1) where $1=$2 limit 3
```

## Conclusion:

By using random subsets of data, the BERNOULLI sampling function in PostgreSQL enables users to evaluate big datasets quickly and effectively. This feature speeds up hypothesis testing and data exploration while simultaneously improving query performance. Data engineers and analysts can extract valuable insights from huge datasets without sacrificing computing effectiveness or statistical validity by using BERNOULLI sampling in PostgreSQL.

The BERNOULLI function in PostgreSQL makes sampling, a useful tool in the database toolbox, accessible to users that want to streamline their data analysis operations. This capability will likely become essential for anyone looking to make educated decisions and extract valuable information from their PostgreSQL databases as the volume and complexity of data continue to increase.