How Can I Efficiently Store and Access Frequently Used Sub-selects in PostgreSQL?

Optimizing Frequent Subqueries in PostgreSQL

PostgreSQL users frequently encounter queries relying on repeated subqueries, often aggregating data from related tables. This article explores efficient methods to manage these common subquery patterns, improving performance and code readability. A typical scenario involves retrieving aggregate data from a secondary table based on a foreign key relationship.

The View Approach: A Robust Solution

While directly embedding subqueries as pseudo-columns isn't possible, PostgreSQL's VIEW functionality provides an excellent alternative. A VIEW defines a virtual table based on existing tables and queries, effectively creating a customized view of the data. This avoids modifying the base table structure while achieving the desired result.

Generated Columns (PostgreSQL 11 ): Enhanced Performance

PostgreSQL 11 and later versions offer generated columns (also called virtual columns). These allow defining a column whose value is computed using an expression, mirroring the goal of embedding subqueries. This approach often yields better performance than views.

Emulating Generated Columns in Older Versions

For versions prior to PostgreSQL 11, or when a virtual column approach is preferred, a user-defined function can simulate generated columns. This function takes the table's row type as input and computes the desired value. The function is then linked to a pseudo-column, achieving similar functionality to generated columns.

Practical Implementation

Let's illustrate with an example:

<code class="language-sql">-- Create tables
CREATE TABLE tbl_a (a_id int, col1 int, col2 int);
CREATE TABLE tbl_b (b_id int, a_id int, colx int);

-- Sample data
INSERT INTO tbl_a VALUES (1,1,1), (2,2,2), (3,3,3), (4,4,4);
INSERT INTO tbl_b VALUES
(1,1,5),  (2,1,5),  (3,1,1)
, (4,2,8),  (5,2,8),  (6,2,6)
, (7,3,11), (8,3,11), (9,3,11);

-- Function to emulate the generated column
CREATE FUNCTION col3(tbl_a)
RETURNS int8
LANGUAGE sql STABLE AS
$func$
SELECT sum(colx)
FROM   tbl_b b
WHERE  b.a_id = .a_id
$func$;</code>

Now, access the col3 pseudo-column:

<code class="language-sql">SELECT a_id, col1, col2, tbl_a.col3
FROM   tbl_a;

SELECT *, a.col3 FROM tbl_a a;</code>

Advantages of Generated Columns

Compared to views, generated columns offer:

• Seamless integration: Included in SELECT * queries without explicit mention.
• Dynamic updates: Automatically reflects changes in the base table.
• Optimized performance: Leverages PostgreSQL's query optimizer for efficiency.

Conclusion

While direct embedding of subqueries isn't supported, PostgreSQL provides effective alternatives – views and generated columns – to manage frequently used subqueries. These methods improve query performance and code clarity, leading to more efficient database management. Choose the approach best suited to your PostgreSQL version and performance requirements.

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