The operators associated with an operator class are identified by
"strategy numbers", which serve to identify the semantics of
each operator within the context of its operator class.
For example, B-trees impose a strict ordering on keys, lesser to greater,
and so operators like "less than" and "greater than or equal
to" are interesting with respect to a B-tree.
Because
PostgreSQL allows the user to define operators,
PostgreSQL cannot look at the name of an operator
(e.g., < or >=) and tell what kind of
comparison it is. Instead, the index access method defines a set of
"strategies", which can be thought of as generalized operators.
Each operator class shows which actual operator corresponds to each
strategy for a particular data type and interpretation of the index
semantics.
B-tree indexes define 5 strategies, as shown in Table 14-1.
Table 14-1. B-tree Strategies
| Operation | Strategy Number |
|---|
| less than | 1 |
| less than or equal | 2 |
| equal | 3 |
| greater than or equal | 4 |
| greater than | 5 |
Hash indexes express only bitwise similarity, and so they define only 1
strategy, as shown in Table 14-2.
Table 14-2. Hash Strategies
| Operation | Strategy Number |
|---|
| equal | 1 |
R-tree indexes express rectangle-containment relationships.
They define 8 strategies, as shown in Table 14-3.
Table 14-3. R-tree Strategies
| Operation | Strategy Number |
|---|
| left of | 1 |
| left of or overlapping | 2 |
| overlapping | 3 |
| right of or overlapping | 4 |
| right of | 5 |
| same | 6 |
| contains | 7 |
| contained by | 8 |
GiST indexes are even more flexible: they do not have a fixed set of
strategies at all. Instead, the "consistency" support routine
of a particular GiST operator class interprets the strategy numbers
however it likes.
By the way, the amorderstrategy column
in pg_am tells whether
the access method supports ordered scan. Zero means it doesn't; if it
does, amorderstrategy is the strategy
number that corresponds to the ordering operator. For example, B-tree
has amorderstrategy = 1, which is its
"less than" strategy number.
In short, an operator class must specify a set of operators that express
each of these semantic ideas for the operator class's data type.
