PERCENTILE_APPROX

Description

The PERCENTILE_APPROX function is used to calculate approximate percentiles, primarily for large datasets. Compared to the PERCENTILE function, it has the following features:

1. Memory Efficiency: Uses fixed-size memory, maintaining low memory consumption even when processing high-cardinality columns (large data volumes)
2. Performance Advantage: Suitable for processing large-scale datasets with faster computation
3. Adjustable Precision: Balance between precision and performance through the compression parameter

Syntax

PERCENTILE_APPROX(<col>, <p> [, <compression>])

Parameters

Parameter	Description
<col>	The column to calculate percentiles for
<p>	Percentile value, range [0.0, 1.0], e.g., 0.99 represents the 99th percentile
<compression>	Optional parameter, compression level, range [2048, 10000], higher values increase precision but consume more memory. If not specified or out of range, uses 10000

Return Value

Returns a DOUBLE value representing the calculated approximate percentile.

Examples

-- Create sample table
CREATE TABLE response_times (
    request_id INT,
    response_time DECIMAL(10, 2)
) DUPLICATE KEY(`request_id`)
DISTRIBUTED BY HASH(`request_id`) BUCKETS AUTO
PROPERTIES (
    "replication_allocation" = "tag.location.default: 1"
);

-- Insert sample data
INSERT INTO response_times VALUES
(1, 10.5),
(2, 15.2),
(3, 20.1),
(4, 25.8),
(5, 30.3),
(6, 35.7),
(7, 40.2),
(8, 45.9),
(9, 50.4),
(10, 100.6);

-- Calculate 99th percentile using different compression levels
SELECT 
    percentile_approx(response_time, 0.99) as p99_default,          -- Default compression
    percentile_approx(response_time, 0.99, 2048) as p99_fast,       -- Lower compression, faster
    percentile_approx(response_time, 0.99, 10000) as p99_accurate   -- Higher compression, more accurate
FROM response_times;

+-------------------+-------------------+-------------------+
| p99_default       | p99_fast          | p99_accurate      |
+-------------------+-------------------+-------------------+
| 100.5999984741211 | 100.5999984741211 | 100.5999984741211 |
+-------------------+-------------------+-------------------+