implications. The requirements of <structfield>amcanunique</structfield>

are discussed in <xref linkend="index-unique-checks"/>.

The <structfield>amcanmulticol</structfield> flag asserts that the

access method supportsmulticolumn indexes, while

access method supportsmulti-key-column indexes, while

<structfield>amoptionalkey</structfield> asserts that it allows scans

where no indexable restriction clause is given for the first index column.

When <structfield>amcanmulticol</structfield> is false,

conditions.

</para>

<para>

The <structfield>amcaninclude</structfield> flag indicates whether the

access method supports <quote>included</quote> columns, that is it can

store (without processing) additional columns beyond the key column(s).

The requirements of the preceding paragraph apply only to the key

columns. In particular, the combination

of <structfield>amcanmulticol</structfield>=<literal>false</literal>

and <structfield>amcaninclude</structfield>=<literal>true</literal> is

sensible: it means that there can only be one key column, but there can

also be included column(s). Also, included columns must be allowed to be

null, independently of <structfield>amoptionalkey</structfield>.

</para>

</sect1>

<sect1 id="index-functions">

</programlisting>

Check whether the index can support <link

linkend="indexes-index-only-scans"><firstterm>index-only scans</firstterm></link> on

the given column, by returning the indexed column values for an index entry

in the form of an <structname>IndexTuple</structname>. The attribute number

is 1-based, i.e., the first column's attno is 1. Returns true if supported,

else false. If the access method does not support index-only scans at all,

the given column, by returning the column's original indexed value.

The attribute number is 1-based, i.e., the first column's attno is 1.

Returns true if supported, else false.

This function should always return true for included columns

(if those are supported), since there's little point in an included

column that can't be retrieved.

If the access method does not support index-only scans at all,

the <structfield>amcanreturn</structfield> field in its <structname>IndexAmRoutine</structname>

struct can be set to NULL.

</para>

core code does not know how to do that and will return NULL. It may

also be advantageous to implement <literal>AMPROP_RETURNABLE</literal> testing,

if that can be done more cheaply than by opening the index and calling

<structfield>amcanreturn</structfield>, which is the core code's default behavior.

<function>amcanreturn</function>, which is the core code's default behavior.

The default behavior should be satisfactory for all other standard

properties.

</para>

<para>

If the index supports <link linkend="indexes-index-only-scans">index-only

scans</link> (i.e., <function>amcanreturn</function> returns true for it),

scans</link> (i.e., <function>amcanreturn</function> returns true for any

of its columns),

then on success the AM must also check <literal>scan-&gt;xs_want_itup</literal>,

and if that is true it must return the originally indexed data for the

index entry. The data can be returned in the form of an

index entry. Columns for which <function>amcanreturn</function> returns

false can be returned as nulls.

The data can be returned in the form of an

<structname>IndexTuple</structname> pointer stored at <literal>scan-&gt;xs_itup</literal>,

with tuple descriptor <literal>scan-&gt;xs_itupdesc</literal>; or in the form of

a <structname>HeapTuple</structname> pointer stored at <literal>scan-&gt;xs_hitup</literal>,

stmt->indexIncludingParams);

numberOfAttributes=list_length(allIndexParams);

if (numberOfAttributes <=0)

if (numberOfKeyAttributes <=0)

ereport(ERROR,

(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),

errmsg("must specify at least one column")));

(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),

errmsg("access method \"%s\" does not support included columns",

accessMethodName)));

if (numberOfAttributes>1&& !amRoutine->amcanmulticol)

if (numberOfKeyAttributes>1&& !amRoutine->amcanmulticol)

ereport(ERROR,

(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),

errmsg("access method \"%s\" does not support multicolumn indexes",