Is there a way to define a named constant in a PostgreSQL query?

Digging Deeper: Available Techniques for Constants

Crafting IMMUTABLE Functions

Construct IMMUTABLE functions for global constants that can be leveraged across different sessions and queries:

CREATE OR REPLACE FUNCTION public.get_pi()
RETURNS numeric AS $$
BEGIN
    RETURN 3.14;
    -- Hey, it's rounder than the world's flatest pancake.
END;
$$ LANGUAGE plpgsql IMMUTABLE;
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get_pi() ensures consistent return of the constant value, promoting DRY (Don't Repeat Yourself) principle. Be mindful of security with function definitions and always schema-qualify your function calls.

Harnessing TEMP TABLES

For session-limited constants, employ TEMP TABLES:

CREATE TEMP TABLE session_constants AS
SELECT 3.14 AS pi;

-- Pi? More like Pie-yaym. Because it's so deliciously constant.
SELECT pi * radius * radius AS area FROM session_constants, circles;
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Exploring GUC mechanism

Adopt PostgreSQL GUC mechanism to set global and session variables acting like constants. This method is more administrative, requiring changes to PostgreSQL's configuration files.

Applying VALUES in CTEs

Frame your constants in a table using VALUES for dynamic updates:

WITH constants(pi) AS (VALUES (3.14))
SELECT pi * radius * radius AS area FROM constants, circles;
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Advanced Utilization & Optimizations

Maximizing performance with constants

Try these tips to integrate these techniques and optimize for performance:

• Limit unnecessary joins, using constants only when needed.
• Inline simple constants to eliminate overhead of additional CTEs.
• Use constants in indexing expressions or partition keys to speed up data lookup and distribution.

Adopting best practices with constants

Here are some best practices to ensure maintainable, clear code:

• Clear naming reflects the constants' meaning and usage.
• Centralize constants in a dedicated schema if shared over multiple queries.
• Document the purpose and use of constants for others to understand your code.

Ensuring reusability and immutability

Incorporate reusable components into query patterns:

• Use IMMUTABLE properties in functions for unchanging constants.
• TEMP TABLES store data within a single session, faster to access than a CTE.
• Custom configuration parameters through GUCs can be employed for application-wide settings or constants needed across sessions.