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<a href="intro.html"><h2>SQLite C Interface</h2></a><h2>Virtual Table Indexing Information</h2><blockquote><pre>struct sqlite3_index_info {
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int nConstraint; /* Number of entries in aConstraint */
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struct sqlite3_index_constraint {
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int iColumn; /* Column on left-hand side of constraint */
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unsigned char op; /* Constraint operator */
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unsigned char usable; /* True if this constraint is usable */
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int iTermOffset; /* Used internally - xBestIndex should ignore */
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} *aConstraint; /* Table of WHERE clause constraints */
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int nOrderBy; /* Number of terms in the ORDER BY clause */
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struct sqlite3_index_orderby {
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int iColumn; /* Column number */
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unsigned char desc; /* True for DESC. False for ASC. */
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} *aOrderBy; /* The ORDER BY clause */
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struct sqlite3_index_constraint_usage {
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int argvIndex; /* if >0, constraint is part of argv to xFilter */
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unsigned char omit; /* Do not code a test for this constraint */
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int idxNum; /* Number used to identify the index */
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char *idxStr; /* String, possibly obtained from sqlite3_malloc */
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int needToFreeIdxStr; /* Free idxStr using sqlite3_free() if true */
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int orderByConsumed; /* True if output is already ordered */
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double estimatedCost; /* Estimated cost of using this index */
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</pre></blockquote><p>
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The sqlite3_index_info structure and its substructures is used as part
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of the <a href="../vtab.html">virtual table</a> interface to
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pass information into and receive the reply from the <a href="../vtab.html#xbestindex">xBestIndex</a>
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method of a <a href="../c3ref/module.html">virtual table module</a>. The fields under **Inputs** are the
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inputs to xBestIndex and are read-only. xBestIndex inserts its
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results into the **Outputs** fields.</p>
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<p>The aConstraint[] array records WHERE clause constraints of the form:</p>
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<p><blockquote>column OP expr</blockquote></p>
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<p>where OP is =, <, <=, >, or >=. The particular operator is
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stored in aConstraint[].op using one of the
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<a href="../c3ref/c_index_constraint_eq.html">SQLITE_INDEX_CONSTRAINT_ values</a>.
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The index of the column is stored in
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aConstraint[].iColumn. aConstraint[].usable is TRUE if the
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expr on the right-hand side can be evaluated (and thus the constraint
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is usable) and false if it cannot.</p>
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<p>The optimizer automatically inverts terms of the form "expr OP column"
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and makes other simplifications to the WHERE clause in an attempt to
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get as many WHERE clause terms into the form shown above as possible.
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The aConstraint[] array only reports WHERE clause terms that are
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relevant to the particular virtual table being queried.</p>
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<p>Information about the ORDER BY clause is stored in aOrderBy[].
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Each term of aOrderBy records a column of the ORDER BY clause.</p>
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<p>The <a href="../vtab.html#xbestindex">xBestIndex</a> method must fill aConstraintUsage[] with information
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about what parameters to pass to xFilter. If argvIndex>0 then
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the right-hand side of the corresponding aConstraint[] is evaluated
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and becomes the argvIndex-th entry in argv. If aConstraintUsage[].omit
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is true, then the constraint is assumed to be fully handled by the
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virtual table and is not checked again by SQLite.</p>
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<p>The idxNum and idxPtr values are recorded and passed into the
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<a href="../vtab.html#xfilter">xFilter</a> method.
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<a href="../c3ref/free.html">sqlite3_free()</a> is used to free idxPtr if and only if
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needToFreeIdxPtr is true.</p>
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<p>The orderByConsumed means that output from <a href="../vtab.html#xfilter">xFilter</a>/<a href="../vtab.html#xnext">xNext</a> will occur in
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the correct order to satisfy the ORDER BY clause so that no separate
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sorting step is required.</p>
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<p>The estimatedCost value is an estimate of the cost of doing the
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particular lookup. A full scan of a table with N entries should have
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a cost of N. A binary search of a table of N entries should have a
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cost of approximately log(N).
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</p><p>See also lists of
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<a href="objlist.html">Objects</a>,
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<a href="constlist.html">Constants</a>, and
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<a href="funclist.html">Functions</a>.</p>
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