65.4. Implementation

63.4.1. SP-GiST Limits

Individual 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 radix 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 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.

63.4.2. 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 picksplit function can return NULL for the nodeLabels array, and likewise the choose function can return NULL for the prefixNodeLabels array during a spgSplitTupleaction. This will in turn result in nodeLabels being NULL during subsequent calls to choose and . In principle, node labels could be used for some inner tuples and omitted for others in the same index.

When working with an inner tuple having unlabeled nodes, it is an error for choose to return spgAddNode, since the set of nodes is supposed to be fixed in such cases.

63.4.3. “All-the-same” Inner Tuples

When dealing with an tuple, a choose result of spgMatchNode is interpreted to mean that the new value can be assigned to any of the equivalent nodes; the core code will ignore the supplied nodeN value and descend into one of the nodes at random (so as to keep the tree balanced). It is an error for choose to return spgAddNode, since that would make the nodes not all equivalent; the spgSplitTuple action must be used if the value to be inserted doesn’t match the existing nodes.

When dealing with an allTheSame tuple, the inner_consistent function should return either all or none of the nodes as targets for continuing the index search, since they are all equivalent. This may or may not require any special-case code, depending on how much the function normally assumes about the meaning of the nodes.