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$Header: /cvsroot/pgsql/doc/src/sgml/xindex.sgml,v 1.10 2000/03/28 02:59:55 tgl Exp $

Postgres documentation

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impose a strict ordering on keys, lesser to greater. Since

<productname>Postgres</productname> allows the user to define operators,

<productname>Postgres</productname> cannot look at the name of an operator

(eg, ">" or "<") and tell what kind of comparison it is. In fact,

(eg, "&gt;" or "&lt;") and tell what kind of comparison it is. In fact,

some access methods don't impose any ordering at all. For example,

<acronym>R-tree</acronym>s express a rectangle-containment relationship,

whereas a hashed data structure expresses only bitwise similarity based

on the value of a hash function. <productname>Postgres</productname>

needs some consistent way of taking a qualification in your query,

looking at the operator and then deciding if a usable index exists. This

implies that <productname>Postgres</productname> needs to know, for

example, that the "<=" and ">" operators partition a

example, that the "&lt;=" and "&gt;" operators partition a

<acronym>B-tree</acronym>. <productname>Postgres</productname>

uses strategies to express these relationships between

operators and the way they can be used to scan indices.

<note>

<para>

Strictly speaking, this routine can return a negative

number (< 0), 0, or a non-zero positive number (> 0).

number (&lt; 0), 0, or a non-zero positive number (&gt; 0).

</para>

</note>

</para>

FROM pg_am am, pg_opclass opcl, complex_ops_tmp c

WHERE amname = 'btree' AND

opcname = 'complex_abs_ops' AND

c.oprname = '<';

c.oprname = '&lt;';

</programlisting>

Now do this for the other operators substituting for the "1" in the

third line above and the "<" in the last line. Note the order:

third line above and the "&lt;" in the last line. Note the order:

"less than" is 1, "less than or equal" is 2, "equal" is 3, "greater

than or equal" is 4, and "greater than" is 5.

</para>