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54.3. ImplementationThis section covers implementation details and other tricks that are useful for implementers of SP-GiST operator classes to know. 54.3.1. SP-GiST LimitsIndividual leaf tuples and inner tuples must fit on a single index page (8KB by default). Therefore, when indexing values of variable-length data types, long values can only be supported by methods such as suffix trees, in which each level of the tree includes a prefix that is short enough to fit on a page, and the final leaf level includes a suffix also short enough to fit on a page. The operator class should set longValuesOK to TRUE only if it is prepared to arrange for this to happen. Otherwise, the SP-GiST core will reject any request to index a value that is too large to fit on an index page. Likewise, it is the operator class's responsibility that inner tuples do not grow too large to fit on an index page; this limits the number of child nodes that can be used in one inner tuple, as well as the maximum size of a prefix value. Another limitation is that when an inner tuple's node points to a set
of leaf tuples, those tuples must all be in the same index page.
(This is a design decision to reduce seeking and save space in the
links that chain such tuples together.) If the set of leaf tuples
grows too large for a page, a split is performed and an intermediate
inner tuple is inserted. For this to fix the problem, the new inner
tuple must divide the set of leaf values into more than one
node group. If the operator class's 54.3.2. SP-GiST Without Node Labels Some tree algorithms use a fixed set of nodes for each inner tuple;
for example, in a quad-tree there are always exactly four nodes
corresponding to the four quadrants around the inner tuple's centroid
point. In such a case the code typically works with the nodes by
number, and there is no need for explicit node labels. To suppress
node labels (and thereby save some space), the When working with an inner tuple having unlabeled nodes, it is an error
for 54.3.3. "All-the-same" Inner Tuples The SP-GiST core can override the results of the
operator class's When dealing with an allTheSame tuple, a When dealing with an allTheSame tuple, the
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