Add SQL-accessible functions for inspecting index AM properties.

Per discussion, we should provide such functions to replace the lost
ability to discover AM properties by inspecting pg_am (cf commit
65c5fcd35).  The added functionality is also meant to displace any code
that was looking directly at pg_index.indoption, since we'd rather not
believe that the bit meanings in that field are part of any client API
contract.

As future-proofing, define the SQL API to not assume that properties that
are currently AM-wide or index-wide will remain so unless they logically
must be; instead, expose them only when inquiring about a specific index
or even specific index column.  Also provide the ability for an index
AM to override the behavior.

In passing, document pg_am.amtype, overlooked in commit 473b93287.

Andrew Gierth, with kibitzing by me and others

Discussion: <87mvl5on7n.fsf@news-spur.riddles.org.uk>

doc/src/sgml/catalogs.sgml

@@ -529,10 +529,11 @@
   </indexterm>
 
   <para>
-   The catalog <structname>pg_am</structname> stores information about index
-   access methods.  There is one row for each index access method supported by
-   the system.  The requirements for index access methods are discussed in
-   detail in <xref linkend="indexam">.
+   The catalog <structname>pg_am</structname> stores information about
+   relation access methods.  There is one row for each access method supported
+   by the system.
+   Currently, only indexes have access methods.  The requirements for index
+   access methods are discussed in detail in <xref linkend="indexam">.
   </para>
 
   <table>
@@ -573,10 +574,30 @@
       </entry>
      </row>
 
+     <row>
+      <entry><structfield>amtype</structfield></entry>
+      <entry><type>char</type></entry>
+      <entry></entry>
+      <entry>
+       Currently always <literal>i</literal> to indicate an index access
+       method; other values may be allowed in future
+      </entry>
+     </row>
     </tbody>
    </tgroup>
   </table>
 
+  <note>
+   <para>
+    Before <productname>PostgreSQL</> 9.6, <structname>pg_am</structname>
+    contained many additional columns representing properties of index access
+    methods.  That data is now only directly visible at the C code level.
+    However, <function>pg_index_column_has_property()</function> and related
+    functions have been added to allow SQL queries to inspect index access
+    method properties; see <xref linkend="functions-info-catalog-table">.
+   </para>
+  </note>
+
  </sect1>
 
 

doc/src/sgml/func.sgml

@@ -16289,6 +16289,18 @@ SELECT pg_type_is_visible('myschema.widget'::regtype);
     <primary>pg_get_viewdef</primary>
    </indexterm>
 
+   <indexterm>
+    <primary>pg_index_column_has_property</primary>
+   </indexterm>
+
+   <indexterm>
+    <primary>pg_index_has_property</primary>
+   </indexterm>
+
+   <indexterm>
+    <primary>pg_indexam_has_property</primary>
+   </indexterm>
+
    <indexterm>
     <primary>pg_options_to_table</primary>
    </indexterm>
@@ -16476,6 +16488,21 @@ SELECT pg_type_is_visible('myschema.widget'::regtype);
               materialized view; lines with fields are wrapped to specified
               number of columns, pretty-printing is implied</entry>
       </row>
+      <row>
+       <entry><literal><function>pg_index_column_has_property(<parameter>index_oid</parameter>, <parameter>column_no</>, <parameter>prop_name</>)</function></literal></entry>
+       <entry><type>boolean</type></entry>
+       <entry>test whether an index column has a specified property</entry>
+      </row>
+      <row>
+       <entry><literal><function>pg_index_has_property(<parameter>index_oid</parameter>, <parameter>prop_name</>)</function></literal></entry>
+       <entry><type>boolean</type></entry>
+       <entry>test whether an index has a specified property</entry>
+      </row>
+      <row>
+       <entry><literal><function>pg_indexam_has_property(<parameter>am_oid</parameter>, <parameter>prop_name</>)</function></literal></entry>
+       <entry><type>boolean</type></entry>
+       <entry>test whether an index access method has a specified property</entry>
+      </row>
       <row>
        <entry><literal><function>pg_options_to_table(<parameter>reloptions</parameter>)</function></literal></entry>
        <entry><type>setof record</type></entry>
@@ -16619,6 +16646,144 @@ SELECT pg_type_is_visible('myschema.widget'::regtype);
    its OID.
   </para>
 
+  <para>
+   <function>pg_index_column_has_property</function>,
+   <function>pg_index_has_property</function>, and
+   <function>pg_indexam_has_property</function> return whether the
+   specified index column, index, or index access method possesses the named
+   property. <literal>NULL</literal> is returned if the property name is not
+   known or does not apply to the particular object, or if the OID or column
+   number does not identify a valid object.  Refer to
+   <xref linkend="functions-info-index-column-props"> for column properties,
+   <xref linkend="functions-info-index-props"> for index properties, and
+   <xref linkend="functions-info-indexam-props"> for access method properties.
+   (Note that extension access methods can define additional property names
+   for their indexes.)
+  </para>
+
+  <table id="functions-info-index-column-props">
+   <title>Index Column Properties</title>
+   <tgroup cols="2">
+    <thead>
+     <row><entry>Name</entry><entry>Description</entry></row>
+    </thead>
+    <tbody>
+     <row>
+      <entry><literal>asc</literal></entry>
+      <entry>Does the column sort in ascending order on a forward scan?
+      </entry>
+     </row>
+     <row>
+      <entry><literal>desc</literal></entry>
+      <entry>Does the column sort in descending order on a forward scan?
+      </entry>
+     </row>
+     <row>
+      <entry><literal>nulls_first</literal></entry>
+      <entry>Does the column sort with nulls first on a forward scan?
+      </entry>
+     </row>
+     <row>
+      <entry><literal>nulls_last</literal></entry>
+      <entry>Does the column sort with nulls last on a forward scan?
+      </entry>
+     </row>
+     <row>
+      <entry><literal>orderable</literal></entry>
+      <entry>Does the column possess any defined sort ordering?
+      </entry>
+     </row>
+     <row>
+      <entry><literal>distance_orderable</literal></entry>
+      <entry>Can the column be scanned in order by a <quote>distance</>
+      operator, for example <literal>ORDER BY col &lt;-&gt; constant</> ?
+      </entry>
+     </row>
+     <row>
+      <entry><literal>returnable</literal></entry>
+      <entry>Can the column value be returned by an index-only scan?
+      </entry>
+     </row>
+     <row>
+      <entry><literal>search_array</literal></entry>
+      <entry>Does the column natively support <literal>col = ANY(array)</>
+      searches?
+      </entry>
+     </row>
+     <row>
+      <entry><literal>search_nulls</literal></entry>
+      <entry>Does the column support <literal>IS NULL</> and
+      <literal>IS NOT NULL</> searches?
+      </entry>
+     </row>
+    </tbody>
+   </tgroup>
+  </table>
+
+  <table id="functions-info-index-props">
+   <title>Index Properties</title>
+   <tgroup cols="2">
+    <thead>
+     <row><entry>Name</entry><entry>Description</entry></row>
+    </thead>
+    <tbody>
+     <row>
+      <entry><literal>clusterable</literal></entry>
+      <entry>Can the index be used in a <literal>CLUSTER</> command?
+      </entry>
+     </row>
+     <row>
+      <entry><literal>index_scan</literal></entry>
+      <entry>Does the index support plain (non-bitmap) scans?
+      </entry>
+     </row>
+     <row>
+      <entry><literal>bitmap_scan</literal></entry>
+      <entry>Does the index support bitmap scans?
+      </entry>
+     </row>
+     <row>
+      <entry><literal>backward_scan</literal></entry>
+      <entry>Can the index be scanned backwards?
+      </entry>
+     </row>
+    </tbody>
+   </tgroup>
+  </table>
+
+  <table id="functions-info-indexam-props">
+   <title>Index Access Method Properties</title>
+   <tgroup cols="2">
+    <thead>
+     <row><entry>Name</entry><entry>Description</entry></row>
+    </thead>
+    <tbody>
+     <row>
+      <entry><literal>can_order</literal></entry>
+      <entry>Does the access method support <literal>ASC</>,
+      <literal>DESC</> and related keywords in
+      <literal>CREATE INDEX</>?
+      </entry>
+     </row>
+     <row>
+      <entry><literal>can_unique</literal></entry>
+      <entry>Does the access method support unique indexes?
+      </entry>
+     </row>
+     <row>
+      <entry><literal>can_multi_col</literal></entry>
+      <entry>Does the access method support indexes with multiple columns?
+      </entry>
+     </row>
+     <row>
+      <entry><literal>can_exclude</literal></entry>
+      <entry>Does the access method support exclusion constraints?
+      </entry>
+     </row>
+    </tbody>
+   </tgroup>
+  </table>
+
   <para>
    <function>pg_options_to_table</function> returns the set of storage
    option name/value pairs

doc/src/sgml/indexam.sgml

@@ -129,6 +129,7 @@ typedef struct IndexAmRoutine
     amcanreturn_function amcanreturn;   /* can be NULL */
     amcostestimate_function amcostestimate;
     amoptions_function amoptions;
+    amproperty_function amproperty;     /* can be NULL */
     amvalidate_function amvalidate;
     ambeginscan_function ambeginscan;
     amrescan_function amrescan;
@@ -407,6 +408,55 @@ amoptions (ArrayType *reloptions,
   <para>
 <programlisting>
 bool
+amproperty (Oid index_oid, int attno,
+            IndexAMProperty prop, const char *propname,
+            bool *res, bool *isnull);
+</programlisting>
+   The <function>amproperty</> method allows index access methods to override
+   the default behavior of <function>pg_index_column_has_property</function>
+   and related functions.
+   If the access method does not have any special behavior for index property
+   inquiries, the <structfield>amproperty</> field in
+   its <structname>IndexAmRoutine</> struct can be set to NULL.
+   Otherwise, the <function>amproperty</> method will be called with
+   <parameter>index_oid</> and <parameter>attno</> both zero for
+   <function>pg_indexam_has_property</function> calls,
+   or with <parameter>index_oid</> valid and <parameter>attno</> zero for
+   <function>pg_index_has_property</function> calls,
+   or with <parameter>index_oid</> valid and <parameter>attno</> greater than
+   zero for <function>pg_index_column_has_property</function> calls.
+   <parameter>prop</> is an enum value identifying the property being tested,
+   while <parameter>propname</> is the original property name string.
+   If the core code does not recognize the property name
+   then <parameter>prop</> is <literal>AMPROP_UNKNOWN</>.
+   Access methods can define custom property names by
+   checking <parameter>propname</> for a match (use <function>pg_strcasecmp</>
+   to match, for consistency with the core code); for names known to the core
+   code, it's better to inspect <parameter>prop</>.
+   If the <structfield>amproperty</> method returns <literal>true</> then
+   it has determined the property test result: it must set <literal>*res</>
+   to the boolean value to return, or set <literal>*isnull</>
+   to <literal>true</> to return a NULL.  (Both of the referenced variables
+   are initialized to <literal>false</> before the call.)
+   If the <structfield>amproperty</> method returns <literal>false</> then
+   the core code will proceed with its normal logic for determining the
+   property test result.
+  </para>
+
+  <para>
+   Access methods that support ordering operators should
+   implement <literal>AMPROP_DISTANCE_ORDERABLE</> property testing, as the
+   core code does not know how to do that and will return NULL.  It may
+   also be advantageous to implement <literal>AMPROP_RETURNABLE</> testing,
+   if that can be done more cheaply than by opening the index and calling
+   <structfield>amcanreturn</>, which is the core code's default behavior.
+   The default behavior should be satisfactory for all other standard
+   properties.
+  </para>
+
+  <para>
+<programlisting>
+bool
 amvalidate (Oid opclassoid);
 </programlisting>
    Validate the catalog entries for the specified operator class, so far as