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Allow GiST distance function to return merely a lower-bound.

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The distance function can now set *recheck = false, like index quals. The
executor will then re-check the ORDER BY expressions, and use a queue to
reorder the results on the fly.

This makes it possible to do kNN-searches on polygons and circles, which
don't store the exact value in the index, but just a bounding box.

Alexander Korotkov and me
This commit is contained in:
Heikki Linnakangas
2015-05-15 14:26:51 +03:00
parent ecd222e770
commit 35fcb1b3d0
19 changed files with 699 additions and 40 deletions
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35
doc/src/sgml/gist.sgml
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@ -105,6 +105,7 @@
<literal>~=</>
</entry>
<entry>
<literal>&lt;-&gt;</>
</entry>
</row>
<row>
@ -163,6 +164,7 @@
<literal>~=</>
</entry>
<entry>
<literal>&lt;-&gt;</>
</entry>
</row>
<row>
@ -206,6 +208,12 @@
</tgroup>
</table>
<para>
Currently, ordering by the distance operator <literal>&lt;-&gt;</>
is supported only with <literal>point</> by the operator classes
of the geometric types.
</para>
<para>
For historical reasons, the <literal>inet_ops</> operator class is
not the default class for types <type>inet</> and <type>cidr</>.
@ -780,6 +788,7 @@ my_distance(PG_FUNCTION_ARGS)
data_type *query = PG_GETARG_DATA_TYPE_P(1);
StrategyNumber strategy = (StrategyNumber) PG_GETARG_UINT16(2);
/* Oid subtype = PG_GETARG_OID(3); */
/* bool *recheck = (bool *) PG_GETARG_POINTER(4); */
data_type *key = DatumGetDataType(entry-&gt;key);
double retval;
@ -792,14 +801,24 @@ my_distance(PG_FUNCTION_ARGS)
</programlisting>
The arguments to the <function>distance</> function are identical to
the arguments of the <function>consistent</> function, except that no
recheck flag is used. The distance to a leaf index entry must always
be determined exactly, since there is no way to re-order the tuples
once they are returned. Some approximation is allowed when determining
the distance to an internal tree node, so long as the result is never
greater than any child's actual distance. Thus, for example, distance
to a bounding box is usually sufficient in geometric applications. The
result value can be any finite <type>float8</> value. (Infinity and
the arguments of the <function>consistent</> function.
</para>
<para>
Some approximation is allowed when determining the distance, as long as
the result is never greater than the entry's actual distance. Thus, for
example, distance to a bounding box is usually sufficient in geometric
applications. For an internal tree node, the distance returned must not
be greater than the distance to any of the child nodes. If the returned
distance is not accurate, the function must set *recheck to false. (This
is not necessary for internal tree nodes; for them, the calculation is
always assumed to be inaccurate). The executor will calculate the
accurate distance after fetching the tuple from the heap, and reorder
the tuples if necessary.
</para>
<para>
The result value can be any finite <type>float8</> value. (Infinity and
minus infinity are used internally to handle cases such as nulls, so it
is not recommended that <function>distance</> functions return these
values.)