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+<!-- keywords: sqlite3_vtab_distinct -->
+<div class=nosearch>
+<a href="../c3ref/intro.html"><h2>SQLite C Interface</h2></a>
+<h2>Determine if a virtual table query is DISTINCT</h2>
+</div>
+<blockquote><pre>
+int sqlite3_vtab_distinct(sqlite3_index_info*);
+</pre></blockquote>
+<p>
+This API may only be used from within an <a href="../vtab.html#xbestindex">xBestIndex method</a>
+of a <a href="../vtab.html">virtual table</a> implementation. The result of calling this
+interface from outside of xBestIndex() is undefined and probably harmful.</p>
+
+<p>The sqlite3_vtab_distinct() interface returns an integer between 0 and
+3.  The integer returned by sqlite3_vtab_distinct()
+gives the virtual table additional information about how the query
+planner wants the output to be ordered. As long as the virtual table
+can meet the ordering requirements of the query planner, it may set
+the "orderByConsumed" flag.</p>
+
+<p><ol><li value="0"><p>
+If the sqlite3_vtab_distinct() interface returns 0, that means
+that the query planner needs the virtual table to return all rows in the
+sort order defined by the "nOrderBy" and "aOrderBy" fields of the
+<a href="../c3ref/index_info.html">sqlite3_index_info</a> object.  This is the default expectation.  If the
+virtual table outputs all rows in sorted order, then it is always safe for
+the xBestIndex method to set the "orderByConsumed" flag, regardless of
+the return value from sqlite3_vtab_distinct().
+<li value="1"><p>
+If the sqlite3_vtab_distinct() interface returns 1, that means
+that the query planner does not need the rows to be returned in sorted order
+as long as all rows with the same values in all columns identified by the
+"aOrderBy" field are adjacent.  This mode is used when the query planner
+is doing a GROUP BY.
+<li value="2"><p>
+If the sqlite3_vtab_distinct() interface returns 2, that means
+that the query planner does not need the rows returned in any particular
+order, as long as rows with the same values in all "aOrderBy" columns
+are adjacent.  Furthermore, only a single row for each particular
+combination of values in the columns identified by the "aOrderBy" field
+needs to be returned.  It is always ok for two or more rows with the same
+values in all "aOrderBy" columns to be returned, as long as all such rows
+are adjacent.  The virtual table may, if it chooses, omit extra rows
+that have the same value for all columns identified by "aOrderBy".
+However omitting the extra rows is optional.
+This mode is used for a DISTINCT query.
+<li value="3"><p>
+If the sqlite3_vtab_distinct() interface returns 3, that means
+that the query planner needs only distinct rows but it does need the
+rows to be sorted. The virtual table implementation is free to omit
+rows that are identical in all aOrderBy columns, if it wants to, but
+it is not required to omit any rows.  This mode is used for queries
+that have both DISTINCT and ORDER BY clauses.
+</ol></p>
+
+<p>For the purposes of comparing virtual table output values to see if the
+values are same value for sorting purposes, two NULL values are considered
+to be the same.  In other words, the comparison operator is "IS"
+(or "IS NOT DISTINCT FROM") and not "==".</p>
+
+<p>If a virtual table implementation is unable to meet the requirements
+specified above, then it must not set the "orderByConsumed" flag in the
+<a href="../c3ref/index_info.html">sqlite3_index_info</a> object or an incorrect answer may result.</p>
+
+<p>A virtual table implementation is always free to return rows in any order
+it wants, as long as the "orderByConsumed" flag is not set.  When the
+the "orderByConsumed" flag is unset, the query planner will add extra
+<a href="../opcode.html">bytecode</a> to ensure that the final results returned by the SQL query are
+ordered correctly.  The use of the "orderByConsumed" flag and the
+sqlite3_vtab_distinct() interface is merely an optimization.  Careful
+use of the sqlite3_vtab_distinct() interface and the "orderByConsumed"
+flag might help queries against a virtual table to run faster.  Being
+overly aggressive and setting the "orderByConsumed" flag when it is not
+valid to do so, on the other hand, might cause SQLite to return incorrect
+results.
+</p><p>See also lists of
+  <a href="../c3ref/objlist.html">Objects</a>,
+  <a href="../c3ref/constlist.html">Constants</a>, and
+  <a href="../c3ref/funclist.html">Functions</a>.</p>
+