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Introduction GIN GIN (Generalized Inverted Index) also uses a B-tree data structure, so it shares similar characteristics with B-Tree. However, unlike B-tree, which stores each value along with a Tid to a single row in the main table, GIN stores the value along with information to access multiple rows sharing that same value (referred to as a Posting List). Therefore, the difference in usage is that B-tree is used to search for a row containing a specific value, while GIN is used to search for small values nested inside a row. Posting List You can understand it as a list containing IDs, which are primary keys corresponding to each row in the main table. If the table does not have an ID, it will use a physical identifier called ctid (Column Tuple Identifier) created by Postgres, with a structure consisting of (Page number, row index within the Page). Based on this ctid, Postgres can access any row in the main table. However, as you know, data in the heap is arranged arbitrarily and when...

Introduction Index When creating an Index, PostgreSQL creates a separate physical file on the disk. Each index has a Relfilenode which is a unique identifier number. An index does not store the entire information of a row but only contains Index Entries, each entry including: Key: The value of the column you index. TID (Tuple Identifier): A physical pointer consisting of a BlockNumber and an OffsetNumber, used for reference to point to the location of that row in the main table (HEAP). When executing a query, the Query Planner Cost Model in Postgres will calculate to choose between reading data from the index or retrieving it directly from the HEAP (Sequential Scan). {Index Scan Cost} = {Index file reading cost} + {Random block reading cost in the table} {Seq Scan Cost} = {Sequential block reading cost in the table} Because the Index only contains the TID to point to the data in the main table, after loading the index content and finding the necessary values, it must perform random I...