I bought a book from crossword; he packed the book and added two bookmarks into my pack. A thought came to my mind. Why do I need a bookmark? I can easily memorize the page number and the next time resume from the same page when I resume reading, or read them all over to reach to the point where I stopped reading. But not all have a blessed memory; moreover, there are better things to remember, my grandpa would rather bookmark and rely on it to help him resume reading. It’s a kind of simple index, isn’t it?
This article focuses on how MS SQL Server uses indexes to read and write data. Data is arranged by SQL Server in the form of extents and pages. Each extent is of size 64 KB, having 8 pages of 8KB sizes. An extent may have data from multiple or same table, but each page holds data from a single table only. Logically, data is stored in record sets in the table. We have fields (columns) identifying the type of data contained in each of the record sets. A table is nothing but a collection of record sets; by default, rows are stored in the form of heaps unless a clustered index has been defined on the table, in which case, record sets are sorted and stored on the clustered index. The heaps structure is a simple arrangement where the inserted record is stored in the next available space on the table page.
Heaps seem a great option when the motive is simply storing data, but when data retrieval steps in, this option back fires. An index acts as a fire fighter in this scenario. Indexes are arranged in the form of a B-Tree where the leaf node holds the data or a pointer to the data. Since the stored data is in a sorted order, indexes precisely know which record is sitting where. Hence an index optimizes and enhances the data retrieval immensely.
But everything comes at a cost; the price we pay for having an index on the table is, each time there is an Insert/Update/Delete, SQL Server updates the active indexes on the table where these DML are operated. Hence simply creating indexes madly for the sake of better data retrieval will not serve the purpose. If there are 20 indexes on a table, each time a DML is done on the table, all these 20 indexes shall be updated so that they can uniquely figure out the location of the record. Let’s dive deep into the indexes.