8th November 2018: PostgreSQL 11.1, 10.6, 9.6.11, 9.5.15, 9.4.20, and 9.3.25 Released!

Sometimes, strategies aren't enough information for the system to figure out how to use an index. Some 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.

In order to manage diverse support routines consistently
across all PostgreSQL access
methods, `pg_am`

includes a column
called `amsupport`. This column
records the number of support routines used by an access method.
For B-trees, this number is one: the routine to take two keys and
return -1, 0, or +1, depending on whether the first key is less
than, equal to, or greater than the second. (Strictly speaking,
this routine can return a negative number (< 0), zero, or a
non-zero positive number (> 0).)

The `amstrategies` entry in
`pg_am`

is just the number of
strategies defined for the access method in question. The
operators for less than, less equal, and so on don't appear in
`pg_am`

. Similarly, `amsupport` is just the number of support
routines required by the access method. The actual routines are
listed elsewhere.

By the way, the `amorderstrategy`
column tells whether the access method supports ordered scan.
Zero means it doesn't; if it does, `amorderstrategy` is the number of the strategy
routine that corresponds to the ordering operator. For example,
B-tree has `amorderstrategy` = 1,
which is its "less than" strategy
number.