Search results
I want to add column to selected columns that will represent the index of the record. Example: id name rating. 1 a 4. 2 b 2. 3 c 8. 4 d 5. For this table I want to get: id name rating index.
The index lists all of the values from the third column, in some meaningful order (say, alphabetically) and for each of them, provides a list of row numbers where that value appears. Now you have a good strategy for finding all the rows where the value of the third column is "M".
Indexing only a prefix of column values in this way can make the index file much smaller. When you index a BLOB or TEXT column, you must specify a prefix length for the index. For example: CREATE TABLE test (blob_col BLOB, INDEX(blob_col(10))); Prefixes can be up to 767 bytes long for InnoDB tables that use the REDUNDANT or COMPACT row format.
MySQL uses indexes to rapidly locate rows with specific column values. Without an index, MySQL must scan the entire table to find the relevant rows. The larger the table, the slower the search becomes.
MySQL can create composite indexes (that is, indexes on multiple columns). An index may consist of up to 16 columns. For certain data types, you can index a prefix of the column (see Section 10.3.5, “Column Indexes”).
Composite indexes are indexes that involve more than one column. Define composite indexes when your queries involve conditions or sorting on multiple columns. Using composite indexes properly can significantly improve the performance of queries that filter or sort based on the indexed columns.
With col_name(N) syntax in an index specification for a string column, you can create an index that uses only the first N characters of the column. Indexing only a prefix of column values in this way can make the index file much smaller.
