As shown in Table 37.8, btree defines one required and two optional support functions.
For each combination of data types that a btree operator family provides comparison operators for, it must provide a comparison support function, registered in pg_amproc
with support function number 1 and amproclefttype
/amprocrighttype
equal to the left and right data types for the comparison (i.e., the same data types that the matching operators are registered with in pg_amop
). The comparison function must take two nonnull values A
and B
and return an int32
value that is <
0
, 0
, or >
0
when A
<
B
, A
=
B
, or A
>
B
, respectively. A null result is disallowed: all values of the data type must be comparable. See src/backend/access/nbtree/nbtcompare.c
for examples.
If the compared values are of a collatable data type, the appropriate collation OID will be passed to the comparison support function, using the standard PG_GET_COLLATION()
mechanism.
Optionally, a btree operator family may provide sort support function(s), registered under support function number 2. These functions allow implementing comparisons for sorting purposes in a more efficient way than naively calling the comparison support function. The APIs involved in this are defined in src/include/utils/sortsupport.h
.
Optionally, a btree operator family may provide in_range support function(s), registered under support function number 3. These are not used during btree index operations; rather, they extend the semantics of the operator family so that it can support window clauses containing the RANGE
offset
PRECEDING
and RANGE
offset
FOLLOWING
frame bound types (see Section 4.2.8). Fundamentally, the extra information provided is how to add or subtract an offset
value in a way that is compatible with the family's data ordering.
An in_range
function must have the signature
in_range(val
type1,base
type1,offset
type2,sub
bool,less
bool) returns bool
val
and base
must be of the same type, which is one of the types supported by the operator family (i.e., a type for which it provides an ordering). However, offset
could be of a different type, which might be one otherwise unsupported by the family. An example is that the builtin time_ops
family provides an in_range
function that has offset
of type interval
. A family can provide in_range
functions for any of its supported types and one or more offset
types. Each in_range
function should be entered in pg_amproc
with amproclefttype
equal to type1
and amprocrighttype
equal to type2
.
The essential semantics of an in_range
function depend on the two Boolean flag parameters. It should add or subtract base
and offset
, then compare val
to the result, as follows:
if !
sub
and !
less
, return val
>=
(base
+
offset
)
if !
sub
and less
, return val
<=
(base
+
offset
)
if sub
and !
less
, return val
>=
(base

offset
)
if sub
and less
, return val
<=
(base

offset
)
Before doing so, the function should check the sign of offset
: if it is less than zero, raise error ERRCODE_INVALID_PRECEDING_OR_FOLLOWING_SIZE
(22013) with error text like “invalid preceding or following size in window function”. (This is required by the SQL standard, although nonstandard operator families might perhaps choose to ignore this restriction, since there seems to be little semantic necessity for it.) This requirement is delegated to the in_range
function so that the core code needn't understand what “less than zero” means for a particular data type.
An additional expectation is that in_range
functions should, if practical, avoid throwing an error if base
+
offset
or base

offset
would overflow. The correct comparison result can be determined even if that value would be out of the data type's range. Note that if the data type includes concepts such as “infinity” or “NaN”, extra care may be needed to ensure that in_range
's results agree with the normal sort order of the operator family.
The results of the in_range
function must be consistent with the sort ordering imposed by the operator family. To be precise, given any fixed values of offset
and sub
, then:
If in_range
with less
= true is true for some val1
and base
, it must be true for every val2
<=
val1
with the same base
.
If in_range
with less
= true is false for some val1
and base
, it must be false for every val2
>=
val1
with the same base
.
If in_range
with less
= true is true for some val
and base1
, it must be true for every base2
>=
base1
with the same val
.
If in_range
with less
= true is false for some val
and base1
, it must be false for every base2
<=
base1
with the same val
.
Analogous statements with inverted conditions hold when less
= false.
If the type being ordered (type1
) is collatable, the appropriate collation OID will be passed to the in_range
function, using the standard PG_GET_COLLATION() mechanism.
in_range
functions need not handle NULL inputs, and typically will be marked strict.