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