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><H1
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><A
NAME="SPGIST-IMPLEMENTATION"
>54.3. Implementation</A
></H1
><P
>   This section covers implementation details and other tricks that are
   useful for implementers of <ACRONYM
CLASS="ACRONYM"
>SP-GiST</ACRONYM
> operator classes to
   know.
  </P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="SPGIST-LIMITS"
>54.3.1. SP-GiST Limits</A
></H2
><P
>   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 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
   <TT
CLASS="STRUCTFIELD"
>longValuesOK</TT
> to TRUE only if it is prepared to arrange for
   this to happen.  Otherwise, the <ACRONYM
CLASS="ACRONYM"
>SP-GiST</ACRONYM
> core will
   reject any request to index a value that is too large to fit
   on an index page.
  </P
><P
>   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.
  </P
><P
>   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 <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>must</I
></SPAN
> divide the set of leaf values into more than one
   node group.  If the operator class's <CODE
CLASS="FUNCTION"
>picksplit</CODE
> function
   fails to do that, the <ACRONYM
CLASS="ACRONYM"
>SP-GiST</ACRONYM
> core resorts to
   extraordinary measures described in <A
HREF="spgist-implementation.html#SPGIST-ALL-THE-SAME"
>Section 54.3.3</A
>.
  </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="SPGIST-NULL-LABELS"
>54.3.2. SP-GiST Without Node Labels</A
></H2
><P
>   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 <CODE
CLASS="FUNCTION"
>picksplit</CODE
>
   function can return NULL for the <TT
CLASS="STRUCTFIELD"
>nodeLabels</TT
> array.
   This will in turn result in <TT
CLASS="STRUCTFIELD"
>nodeLabels</TT
> being NULL during
   subsequent calls to <CODE
CLASS="FUNCTION"
>choose</CODE
> and <CODE
CLASS="FUNCTION"
>inner_consistent</CODE
>.
   In principle, node labels could be used for some inner tuples and omitted
   for others in the same index.
  </P
><P
>   When working with an inner tuple having unlabeled nodes, it is an error
   for <CODE
CLASS="FUNCTION"
>choose</CODE
> to return <TT
CLASS="LITERAL"
>spgAddNode</TT
>, since the set
   of nodes is supposed to be fixed in such cases.  Also, there is no
   provision for generating an unlabeled node in <TT
CLASS="LITERAL"
>spgSplitTuple</TT
>
   actions, since it is expected that an <TT
CLASS="LITERAL"
>spgAddNode</TT
> action will
   be needed as well.
  </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="SPGIST-ALL-THE-SAME"
>54.3.3. <SPAN
CLASS="QUOTE"
>"All-the-same"</SPAN
> Inner Tuples</A
></H2
><P
>   The <ACRONYM
CLASS="ACRONYM"
>SP-GiST</ACRONYM
> core can override the results of the
   operator class's <CODE
CLASS="FUNCTION"
>picksplit</CODE
> function when
   <CODE
CLASS="FUNCTION"
>picksplit</CODE
> fails to divide the supplied leaf values into
   at least two node categories.  When this happens, the new inner tuple
   is created with multiple nodes that each have the same label (if any)
   that <CODE
CLASS="FUNCTION"
>picksplit</CODE
> gave to the one node it did use, and the
   leaf values are divided at random among these equivalent nodes.
   The <TT
CLASS="LITERAL"
>allTheSame</TT
> flag is set on the inner tuple to warn the
   <CODE
CLASS="FUNCTION"
>choose</CODE
> and <CODE
CLASS="FUNCTION"
>inner_consistent</CODE
> functions that the
   tuple does not have the node set that they might otherwise expect.
  </P
><P
>   When dealing with an <TT
CLASS="LITERAL"
>allTheSame</TT
> tuple, a <CODE
CLASS="FUNCTION"
>choose</CODE
>
   result of <TT
CLASS="LITERAL"
>spgMatchNode</TT
> is interpreted to mean that the new
   value can be assigned to any of the equivalent nodes; the core code will
   ignore the supplied  <TT
CLASS="STRUCTFIELD"
>nodeN</TT
> value and descend into one
   of the nodes at random (so as to keep the tree balanced).  It is an
   error for <CODE
CLASS="FUNCTION"
>choose</CODE
> to return <TT
CLASS="LITERAL"
>spgAddNode</TT
>, since
   that would make the nodes not all equivalent; the
   <TT
CLASS="LITERAL"
>spgSplitTuple</TT
> action must be used if the value to be inserted
   doesn't match the existing nodes.
  </P
><P
>   When dealing with an <TT
CLASS="LITERAL"
>allTheSame</TT
> tuple, the
   <CODE
CLASS="FUNCTION"
>inner_consistent</CODE
> 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 <CODE
CLASS="FUNCTION"
>inner_consistent</CODE
> function normally
   assumes about the meaning of the nodes.
  </P
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