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Introduction In this article, we will learn about Merge Join. How It Works Simply put, how Merge Join works is like sorting two lists in ascending order first, then placing them side by side for comparison. Assuming we need to compare the id column of two tables, A and B, the specific process is as follows: Step 1: Sort Phase PostgreSQL must ensure that both lists are already sorted by the order of the join column (for example, from smallest to largest). If the two tables are already sorted, such as when the join column is a Primary Key or has an Index, Postgres will skip this step and jump to step 2. Step 2: Merge Phase PostgreSQL will place two pointers starting from the first row of the two tables and begin comparing: If the results on both sides are equal: Return the result, then move the pointer in table B to the next row to continue checking (since data in a column can be duplicated). If the id in table A is smaller: Move the table A pointer to the next row. If the id in table A ...

Introduction In SQL and specifically in PostgreSQL, there are 4 types of Joins as follows: INNER JOIN: Only retrieves records that have a match in both tables. LEFT JOIN: Retrieves all records from the left table, and if there is no match in the right table, the values are set to NULL. RIGHT JOIN: The opposite of LEFT JOIN, rarely used because it can be rewritten in reverse using LEFT JOIN. FULL OUTER JOIN: Retrieves all records from both tables, filling with NULL where there is no match. Nested Loop Join When executing a join, the Postgres Optimizer automatically performs an analysis based on the datasets of the 2 tables to select the most efficient and suitable algorithm for the current situation First, let us look at the Nested Loop Join. This is the most basic join algorithm, and its operational mechanism is very straightforward: it takes each item from one table to compare it with every item from the other table, acting exactly like two nested for loops in programming Use cases Th...