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Since PostgreSQL doesn't support clustered indexes, I'm considering MSSQL server. I've read the article comparing clustered and non-clustered indexes. The gist of the article is that (emphasize mine):
Non clustered indexes store both a value and a pointer to the actual row that holds that value.
And
Clustered indexes don’t need to store a pointer to the actual row because of the fact that the rows in the table are stored on disk in the same exact order as the clustered index
As I was told there and there it was very difficult to support the physical ordering of the table's data, especially if the table is splitted among multiple drives. And now, I meet the clustered index concept assuming that data stored in some order physically. This's what I was confused by.
Question: What is the clustered index structure? Does it support tree-like structure to traverse over, like PosgtreSQL does for btree indexes?
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Clustered indexes sort and store the data rows in the table or view based on their key values. These are the columns included in the index definition. There can be only one clustered index per table, because the data rows themselves can be stored in only one order.
Clustered index is as same as dictionary where the data is arranged by alphabetical order. In clustered index, index contains pointer to block but not direct data. Example of Clustered Index – If you apply primary key to any column, then automatically it will become clustered index.
In a Clustered table, a SQL Server clustered index is used to store the data rows sorted based on the clustered index key values. SQL Server allows us to create only one Clustered index per each table, as the data can be sorted in the table using one order criteria.
As a guideline, clustered Indexes should be Narrow, Unique, Static and Ever Increasing (NUSE). Michelle Ufford Explains why. Clustered indexes are the cornerstone of good database design.
In SQL Server, indexes are organized as B-trees. Each page in an index B-tree is called an index node. The top node of the B-tree is called the root node. The bottom level of nodes in the index is called the leaf nodes. Any index levels between the root and the leaf nodes are collectively known as intermediate levels. In a clustered index, the leaf nodes contain the data pages of the underlying table. The root and intermediate level nodes contain index pages holding index rows. Each index row contains a key value and a pointer to either an intermediate level page in the B-tree, or a data row in the leaf level of the index. The pages in each level of the index are linked in a doubly-linked list.
Clustered indexes have one row in sys.partitions, with index_id = 1 for each partition used by the index. By default, a clustered index has a single partition. When a clustered index has multiple partitions, each partition has a B-tree structure that contains the data for that specific partition. For example, if a clustered index has four partitions, there are four B-tree structures; one in each partition.
for ref.
https://technet.microsoft.com/en-us/library/ms177443(v=sql.105).aspx http://www.sqlservercentral.com/blogs/practicalsqldba/2013/03/14/sql-server-part-4-explaining-the-non-clustered-index-structure-/
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