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Diffstat (limited to 'src/backend/access/nbtree/README')
| -rw-r--r-- | src/backend/access/nbtree/README | 53 |
1 files changed, 0 insertions, 53 deletions
diff --git a/src/backend/access/nbtree/README b/src/backend/access/nbtree/README index a3f11da8d5a..34f78b2f50a 100644 --- a/src/backend/access/nbtree/README +++ b/src/backend/access/nbtree/README @@ -623,56 +623,3 @@ routines must treat it accordingly. The actual key stored in the item is irrelevant, and need not be stored at all. This arrangement corresponds to the fact that an L&Y non-leaf page has one more pointer than key. - -Notes to Operator Class Implementors ------------------------------------- - -With this implementation, we require each supported combination of -datatypes to supply us with a comparison procedure via pg_amproc. -This procedure must take two nonnull values A and B and return an int32 < 0, -0, or > 0 if A < B, A = B, or A > B, respectively. The procedure must -not return INT_MIN for "A < B", since the value may be negated before -being tested for sign. A null result is disallowed, too. See nbtcompare.c -for examples. - -There are some basic assumptions that a btree operator family must satisfy: - -An = operator must be an equivalence relation; that is, for all non-null -values A,B,C of the datatype: - - A = A is true reflexive law - if A = B, then B = A symmetric law - if A = B and B = C, then A = C transitive law - -A < operator must be a strong ordering relation; that is, for all non-null -values A,B,C: - - A < A is false irreflexive law - if A < B and B < C, then A < C transitive law - -Furthermore, the ordering is total; that is, for all non-null values A,B: - - exactly one of A < B, A = B, and B < A is true trichotomy law - -(The trichotomy law justifies the definition of the comparison support -procedure, of course.) - -The other three operators are defined in terms of these two in the obvious way, -and must act consistently with them. - -For an operator family supporting multiple datatypes, the above laws must hold -when A,B,C are taken from any datatypes in the family. The transitive laws -are the trickiest to ensure, as in cross-type situations they represent -statements that the behaviors of two or three different operators are -consistent. As an example, it would not work to put float8 and numeric into -an opfamily, at least not with the current semantics that numerics are -converted to float8 for comparison to a float8. Because of the limited -accuracy of float8, this means there are distinct numeric values that will -compare equal to the same float8 value, and thus the transitive law fails. - -It should be fairly clear why a btree index requires these laws to hold within -a single datatype: without them there is no ordering to arrange the keys with. -Also, index searches using a key of a different datatype require comparisons -to behave sanely across two datatypes. The extensions to three or more -datatypes within a family are not strictly required by the btree index -mechanism itself, but the planner relies on them for optimization purposes. |