10th November 2022:
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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.

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.