How Can PostgreSQL Stored Computed Columns Improve Performance for Complex Queries?

Virtual column to store query

PostgreSQL provides many ways to incorporate complex calculations or subqueries into a table's schema. One approach is to create a view, which provides a virtual representation of the data that can be queried like a normal table. However, the view is not stored in the database and is recalculated for each query, which may reduce efficiency for complex queries that are used frequently.

Another approach is to use a stored computed column, which allows you to define the value of a column based on an expression that contains other columns or external data sources. A stored computed column is physically stored in the table and its value is calculated and updated whenever the underlying data changes.

To use stored computed columns to store a subquery as a pseudo column, you can create a function that wraps the subquery and reference it in the computed column definition. Here's an example of how to do this:

<code class="language-sql">CREATE FUNCTION col3(tbl_a)
RETURNS int8
LANGUAGE SQL STABLE
AS
$$
SELECT sum(colx)
FROM   tbl_b b
WHERE  b.a_id = .a_id
$$;

ALTER TABLE tbl_a ADD COLUMN col3 INT8 STORED AS col3(tbl_a);</code>

This method allows you to query derived columns just like normal columns:

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

Compared with views, the advantage of using stored calculated columns is that the column values ​​are physically stored and do not need to be recalculated for each query, thus improving the performance of frequently used complex queries. Additionally, stored computed columns are fully integrated into the table schema, allowing them to be used for indexes, foreign key constraints, and other database operations.

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