Efficient random row selection method for PostgreSQL
PostgreSQL provides a variety of methods for efficiently selecting random rows.
Method 1: Use Random() and Limit clause
This method uses the random() function and the LIMIT clause:
SELECT * FROM table ORDER BY random() LIMIT 1000;
However, for large tables, this method may be slower as it requires a full table scan.
Method 2: Index-based method
This method uses the primary key index to optimize the query:
WITH params AS (
SELECT 1 AS min_id, -- 最小ID (大于等于當(dāng)前最小ID)
, 5100000 AS id_span -- 四舍五入 (max_id - min_id + 緩沖)
)
SELECT *
FROM (
SELECT p.min_id + trunc(random() * p.id_span)::integer AS id
FROM params p
, generate_series(1, 1100) g -- 1000 + 緩沖
GROUP BY 1 -- 去除重復(fù)項(xiàng)
) r
JOIN table USING (id)
LIMIT 1000; -- 去除多余項(xiàng)
This method is faster than method one because it uses an index scan instead of a full table scan.
Method 3: Use recursive CTE
This method uses a recursive common table expression (CTE) to handle missing values ??in the ID column:
WITH RECURSIVE random_pick AS (
SELECT *
FROM (
SELECT 1 + trunc(random() * 5100000)::int AS id
FROM generate_series(1, 1030) -- 1000 + 百分幾 - 根據(jù)需要調(diào)整
LIMIT 1030 -- 查詢規(guī)劃器提示
) r
JOIN table b USING (id) -- 去除缺失值
UNION -- 去除重復(fù)項(xiàng)
SELECT b.*
FROM (
SELECT 1 + trunc(random() * 5100000)::int AS id
FROM random_pick r -- 加上百分幾 - 根據(jù)需要調(diào)整
LIMIT 999 -- 小于1000，查詢規(guī)劃器提示
) r
JOIN table b USING (id) -- 去除缺失值
)
TABLE random_pick
LIMIT 1000; -- 實(shí)際限制
Method 4: Use TABLESAMPLE SYSTEM (n)
PostgreSQL 9.5 introduced the TABLESAMPLE SYSTEM (n) syntax, where n is a percentage between 0 and 100:
SELECT * FROM big TABLESAMPLE SYSTEM ((1000 * 100) / 5100000.0);
This method is fast, but may not return truly random samples due to clustering effects.
Comparison and suggestions
If the table has few missing values ??for the ID column and the primary key index is in place, Method two (index-based method) is the best choice as it provides the best speed and accuracy sex.
For tables with many missing values, please consider Method 3 (recursive CTE), which can effectively handle missing values.
Method one (random() and limit) has lower performance and should be used with smaller tables.
Method 4(TABLESAMPLE SYSTEM) is fast, but not as accurate as other methods. It can be used to make quick estimates on large tables.
The above is the detailed content of How can I efficiently select random rows in PostgreSQL?. For more information, please follow other related articles on the PHP Chinese website!
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