Tags: record physical default SQL full Clust Advantage limit data
Clustered index A table can have only one, and a table of non-clustered indexes may exist multiple.
Clustered index storage records are physically contiguous, while nonclustered indexes are logically contiguous, and physical storage is not contiguous.
Create clustered index dcity on city (name)//Creating a clustered Index
Create unclustered index dcity on city (name)//creating Nonclustered indexes
First, the advantages:
Besides the disadvantages:
Index Overwrite: The content stored by the index is the final output of the data
SQLite does not support clustered indexes, Android requires a "_id" field by default, which ensures that the data you insert will be inserted in an integer order of "_id", and that the integer type's primary key will play the same role as the clustered index. So do not create an index on the primary key that is declared as: INTEGER PRIMARY key.
Many friends who are unfamiliar with the index create an index in the table but find that it is not in effect, which is mostly about what I'm going to say next. There are some restrictions on the columns that appear in the WHERE clause, which are related to the leading column, for the index to take effect. The so-called leading column is the first column in the creation of a compound index statement or a contiguous number of columns. For example, by creating index comp_ind on table1 (x, y, z), X,XY,XYZ is the leading column, and Yz,y,z is not. These, for other databases, may have some minor differences, here is the standard SQLite. In the WHERE clause, the leading column must use the equals or in operation, and the rightmost column can use inequalities so that the index can take effect completely. Also, the columns in the WHERE clause do not need to be fully indexed, but you must ensure that there are no gaps between the indexed columns.
Talk about clustered indexes, nonclustered indexes and use in SQLite.