11.10. Operator Classes and Operator Families#
An index definition can specify anoperator class for each column of an index.
CREATE INDEXnameONtable(columnopclass[ (opclass_options) ] [sort options] [, ...]);
The operator class identifies the operators to be used by the index for that column. For example, a B-tree index on the typeint4 would use theint4_ops class; this operator class includes comparison functions for values of typeint4. In practice the default operator class for the column's data type is usually sufficient. The main reason for having operator classes is that for some data types, there could be more than one meaningful index behavior. For example, we might want to sort a complex-number data type either by absolute value or by real part. We could do this by defining two operator classes for the data type and then selecting the proper class when making an index. The operator class determines the basic sort ordering (which can then be modified by adding sort optionsCOLLATE,ASC/DESC and/orNULLS FIRST/NULLS LAST).
There are also some built-in operator classes besides the default ones:
The operator classes
text_pattern_ops,varchar_pattern_ops, andbpchar_pattern_opssupport B-tree indexes on the typestext,varchar, andcharrespectively. The difference from the default operator classes is that the values are compared strictly character by character rather than according to the locale-specific collation rules. This makes these operator classes suitable for use by queries involving pattern matching expressions (LIKEor POSIX regular expressions) when the database does not use the standard“C” locale. As an example, you might index avarcharcolumn like this:CREATE INDEX test_index ON test_table (col varchar_pattern_ops);
Note that you should also create an index with the default operator class if you want queries involving ordinary
<,<=,>, or>=comparisons to use an index. Such queries cannot use thexxx_pattern_opsxxx_pattern_ops
The following query shows all defined operator classes:
SELECT am.amname AS index_method, opc.opcname AS opclass_name, opc.opcintype::regtype AS indexed_type, opc.opcdefault AS is_default FROM pg_am am, pg_opclass opc WHERE opc.opcmethod = am.oid ORDER BY index_method, opclass_name;
An operator class is actually just a subset of a larger structure called anoperator family. In cases where several data types have similar behaviors, it is frequently useful to define cross-data-type operators and allow these to work with indexes. To do this, the operator classes for each of the types must be grouped into the same operator family. The cross-type operators are members of the family, but are not associated with any single class within the family.
This expanded version of the previous query shows the operator family each operator class belongs to:
SELECT am.amname AS index_method, opc.opcname AS opclass_name, opf.opfname AS opfamily_name, opc.opcintype::regtype AS indexed_type, opc.opcdefault AS is_default FROM pg_am am, pg_opclass opc, pg_opfamily opf WHERE opc.opcmethod = am.oid AND opc.opcfamily = opf.oid ORDER BY index_method, opclass_name;
This query shows all defined operator families and all the operators included in each family:
SELECT am.amname AS index_method, opf.opfname AS opfamily_name, amop.amopopr::regoperator AS opfamily_operator FROM pg_am am, pg_opfamily opf, pg_amop amop WHERE opf.opfmethod = am.oid AND amop.amopfamily = opf.oid ORDER BY index_method, opfamily_name, opfamily_operator;
Tip
psql has commands\dAc,\dAf, and\dAo, which provide slightly more sophisticated versions of these queries.