Merge Join
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 ...