How Can UNION ALL Optimize Decrementing Precision SELECT Statements in PostgreSQL?

Alternative Approach for Decrementing Precision SELECTs

Using UNION ALL for Efficient Querying

When attempting to search for a row in a table with decrementing precision, it is inefficient to execute multiple SELECT statements one after the other. Instead, consider utilizing a UNION ALL query to combine multiple SELECTs into a single efficient expression.

Optimized Query:

SELECT * FROM image WHERE name = 'name105' AND group_id = 10
UNION ALL
SELECT * FROM image WHERE name = 'name105'
UNION ALL
SELECT * FROM image WHERE group_id = 10
LIMIT 1;

Explanation:

This query utilizes three SELECT statements, each with a progressively lower level of precision:

1. The first SELECT seeks a row that matches both the name and group_id parameters.
2. If no result is found, the second SELECT searches for a row that matches only the name parameter.
3. Finally, the third SELECT searches for a row that matches only the group_id parameter.

By combining these SELECTs using UNION ALL, the query ensures that the first match is returned, even if it is only based on one parameter.

Benefits:

• Optimization: PostgreSQL intelligently optimizes the query, allowing it to terminate early once the required number of rows is obtained.
• Generic Solution: This approach can be used for any number of search parameters.
• Sorting Relevance: While the query returns only one row, it can still be sorted by relevance if the underlying table has an appropriate index.

Note:

In PostgreSQL versions 11 and later, consider the possibility of parallel append when using UNION ALL. This can affect the reliability of the query in certain scenarios. Refer to the thread below for details:

• [Are results from UNION ALL clauses always appended in order?](https://dba.stackexchange.com/questions/239570/are-results-from-union-all-clauses-always-appended-in-order)

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