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Introduction In this article, we will explore another type of index, which is the Hash index. True to its name, when using this index type, Postgres passes the value to be indexed through a hash function to generate a number ranging from 0 to 4,294,697,295 with a uint32 (4 bytes) type. Index Creation Process As soon as you create a Hash index, Postgres pre-allocates Pages to store the indexes. If the table does not have data yet, Postgres will still create: 1 Metapage : Always located at the first position (Block 0). This page stores the configuration of the index. 1 Bitmap page : Used to manage which pages in the file are currently empty to avoid Overflow pages. 10 default Primary Bucket Pages . Thus, right from the start, your newly created Hash Index file will have a size of 12 Pages x 8KB/Page = 96 KB. If the table already contains data, based on the configuration information about the Ram limit that Postgres can use, it will create the maximum number of Pages (that it can create)...

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...