Strategies aren't usually enough information for the system to figure
out how to use an index. In practice, the access methods require
additional support routines in order to work. For example, the B-tree
access method must be able to compare two keys and determine whether one
is greater than, equal to, or less than the other. Similarly, the
R-tree access method must be able to compute
intersections, unions, and sizes of rectangles. These
operations do not correspond to operators used in qualifications in
SQL queries; they are administrative routines used by
the access methods, internally.

Just as with operators, the operator class identifies which specific
functions should play each of these roles for a given data type and
semantic interpretation. The index access method specifies the set
of functions it needs, and the operator class identifies the correct
functions to use by assigning "support function numbers" to them.

B-trees require a single support function, as shown in Table 14-4.

**Table 14-4. B-tree Support Functions**

Function | Support Number |
---|

Compare two keys and return an integer less than zero, zero, or
greater than zero, indicating whether the first key is less than, equal to,
or greater than the second.
| 1 |

Hash indexes likewise require one support function, as shown in Table 14-5.

**Table 14-5. Hash Support Functions**

Function | Support Number |
---|

Compute the hash value for a key | 1 |

R-tree indexes require three support functions,
as shown in Table 14-6.

**Table 14-6. R-tree Support Functions**

Function | Support Number |
---|

union | 1 |

intersection | 2 |

size | 3 |

GiST indexes require seven support functions,
as shown in Table 14-7.

**Table 14-7. GiST Support Functions**

Function | Support Number |
---|

consistent | 1 |

union | 2 |

compress | 3 |

decompress | 4 |

penalty | 5 |

picksplit | 6 |

equal | 7 |