199 lines
7.3 KiB
HTML
199 lines
7.3 KiB
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<!-- keywords: sqlite3_vtab_in -->
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<div class=nosearch>
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<a href="../c3ref/intro.html"><h2>SQLite C Interface</h2></a>
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<h2>Identify and handle IN constraints in xBestIndex</h2>
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</div>
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<blockquote><pre>
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int sqlite3_vtab_in(sqlite3_index_info*, int iCons, int bHandle);
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</pre></blockquote>
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<p>
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This interface may only be used from within an
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<a href="../vtab.html#xbestindex">xBestIndex() method</a> of a <a href="../vtab.html">virtual table</a> implementation.
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The result of invoking this interface from any other context is
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undefined and probably harmful.</p>
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<p>A constraint on a virtual table of the form
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"<a href="../lang_expr.html#in_op">column IN (...)</a>" is
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communicated to the xBestIndex method as a
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<a href="../c3ref/c_index_constraint_eq.html">SQLITE_INDEX_CONSTRAINT_EQ</a> constraint. If xBestIndex wants to use
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this constraint, it must set the corresponding
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aConstraintUsage[].argvIndex to a positive integer. Then, under
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the usual mode of handling IN operators, SQLite generates <a href="../opcode.html">bytecode</a>
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that invokes the <a href="../vtab.html#xfilter">xFilter() method</a> once for each value
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on the right-hand side of the IN operator. Thus the virtual table
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only sees a single value from the right-hand side of the IN operator
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at a time.</p>
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<p>In some cases, however, it would be advantageous for the virtual
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table to see all values on the right-hand of the IN operator all at
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once. The sqlite3_vtab_in() interfaces facilitates this in two ways:</p>
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<p><ol>
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<li><p>
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A call to sqlite3_vtab_in(P,N,-1) will return true (non-zero)
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if and only if the <a href="../c3ref/index_info.html">P->aConstraint</a>[N] constraint
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is an <a href="../lang_expr.html#in_op">IN operator</a> that can be processed all at once. In other words,
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sqlite3_vtab_in() with -1 in the third argument is a mechanism
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by which the virtual table can ask SQLite if all-at-once processing
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of the IN operator is even possible.</p>
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<p><li><p>
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A call to sqlite3_vtab_in(P,N,F) with F==1 or F==0 indicates
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to SQLite that the virtual table does or does not want to process
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the IN operator all-at-once, respectively. Thus when the third
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parameter (F) is non-negative, this interface is the mechanism by
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which the virtual table tells SQLite how it wants to process the
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IN operator.
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</ol></p>
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<p>The sqlite3_vtab_in(P,N,F) interface can be invoked multiple times
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within the same xBestIndex method call. For any given P,N pair,
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the return value from sqlite3_vtab_in(P,N,F) will always be the same
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within the same xBestIndex call. If the interface returns true
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(non-zero), that means that the constraint is an IN operator
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that can be processed all-at-once. If the constraint is not an IN
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operator or cannot be processed all-at-once, then the interface returns
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false.</p>
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<p>All-at-once processing of the IN operator is selected if both of the
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following conditions are met:</p>
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<p><ol>
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<li><p> The P->aConstraintUsage[N].argvIndex value is set to a positive
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integer. This is how the virtual table tells SQLite that it wants to
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use the N-th constraint.</p>
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<p><li><p> The last call to sqlite3_vtab_in(P,N,F) for which F was
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non-negative had F>=1.
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</ol></p>
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<p>If either or both of the conditions above are false, then SQLite uses
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the traditional one-at-a-time processing strategy for the IN constraint.
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If both conditions are true, then the argvIndex-th parameter to the
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xFilter method will be an <a href="../c3ref/value.html">sqlite3_value</a> that appears to be NULL,
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but which can be passed to <a href="../c3ref/vtab_in_first.html">sqlite3_vtab_in_first()</a> and
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<a href="../c3ref/vtab_in_first.html">sqlite3_vtab_in_next()</a> to find all values on the right-hand side
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of the IN constraint.
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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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