What is the purpose of defining an operator class when defining index in postgres?

Question

For example

CREATE INDEX my_index_name
    ON public.my_table USING btree
    (my_column  int8_ops)
    TABLESPACE pg_default;

vs

CREATE INDEX my_index_name
    ON public.my_table USING btree
    (my_column)
    TABLESPACE pg_default;

What is the difference?

Answer 1

As much as I dislike quoting verbatim, I think what is written in the manual describes it most correctly and succintly, and I doubt I could do better:

The operator class identifies the operators to be used by the index for that column. For example, a B-tree index on the type int4 would use the int4_ops class; this operator class includes comparison functions for values of type int4. 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 options COLLATE, ASC/DESC and/or NULLS FIRST/NULLS LAST).

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As for your example, if my_column is defined as type int8, then specifying the operator class as int8_ops is moot, as that would be the default operator for that type.