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What is an index?
A database index is a data structure that improves the speed of data retrieval operations on a database table at the cost of additional writes and storage space to maintain the index data structure. Indexes are used to quickly locate data without having to search every row in a database table every time a database table is accessed.
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What does a PostgreSQL index look like?
When creating an index on the name field(for example), PostgreSQL saves all of them inside a B-Tree - keys are now represented by names. Each entry in the index consists of a C structure called IndexTupleData, and is followed by a bitmap and a value. Bitmaps record if any of the index attributes ...
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Querying using indexes
Suppose we have an index on names and we query the following: SELECT * FROM users WHERE name = 'Captain Nemo' ORDER BY id ASC LIMIT 1. As you can see in the image, Postgre quickly narrows the results down until it finds the specific node where the name is located. A binary search algorithm is the...
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Disadavtanges of indexes
Although indexes are useful as they provide faster searches, having too many of them could turn out to be a problem.
- Space issues: Indexes consume space and if the table the indexes are on are particularly large, it may have an impact.
- Write speed: when inserting, deleting or u...
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B-Trees
PostgreSQL implements several types of indexes, such as btree, hash, gist, spgist. The default and most common type of index is btree. A btree (balanced tree) allows for easier and faster searching. This can be seen in the image above where we search for the key with the value of 53.
Btree...
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Index scans and index-only scans
An example of an index scan is the earlier query. As there is an index on the user column and we asked for all the user details, after the value is found in the index, Postgre then goes to the heap and fetches the other fields.
An index-only scan means that all the needed information is fou...
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Bitmap scans
If you only select a few rows, Postgre will decide on an index scan – if you select a majority of them, Postgre will decide on a table scan. But what if you read too much for an index scan to be efficient but too little for a sequential scan? The solution to the problem is to use a bitmap scan. T...
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