Can UNION ALL with LIMIT 1 Efficiently Simulate Sequential Queries?

Sequential Selective Queries Using UNION ALL

Problem:
Seeking a single row in a database table with progressively reduced search criteria. For example:

SELECT * FROM image WHERE name LIKE 'text' AND group_id = 10 LIMIT 1

If no result is obtained, execute:

SELECT * FROM image WHERE name LIKE 'text' LIMIT 1

If still no result, execute:

SELECT * FROM image WHERE group_id = 10 LIMIT 1

Can this process be performed with a single expression?

Solution:

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:

• UNION ALL combines the results of multiple SELECT statements into a single table.
• The LIMIT clause specifies that only the first row of the combined result should be returned.
• Indexes on (name, group_id) and (group_id) are crucial for performance.
• This query is particularly efficient because PostgreSQL optimizes the execution plan to stop processing additional SELECT statements once the LIMIT is satisfied.

Generic Solution:

The above approach can be generalized for any number of search parameters by adding additional SELECT statements to the UNION ALL chain.

Considerations for Sorted Results:

Since the LIMIT clause applies to the entire result, sorting is not particularly useful because only the first row will be returned.

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