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## SQLite C Interface

## Virtual Table Indexing Information

>     struct sqlite3_index_info {
>       /* Inputs */
>       int nConstraint;           /* Number of entries in aConstraint */
>       struct sqlite3_index_constraint {
>          int iColumn;              /* Column on left-hand side of constraint */
>          unsigned char op;         /* Constraint operator */
>          unsigned char usable;     /* True if this constraint is usable */
>          int iTermOffset;          /* Used internally - xBestIndex should ignore */
>       } *aConstraint;            /* Table of WHERE clause constraints */
>       int nOrderBy;              /* Number of terms in the ORDER BY clause */
>       struct sqlite3_index_orderby {
>          int iColumn;              /* Column number */
>          unsigned char desc;       /* True for DESC.  False for ASC. */
>       } *aOrderBy;               /* The ORDER BY clause */
>       /* Outputs */
>       struct sqlite3_index_constraint_usage {
>         int argvIndex;           /* if >0, constraint is part of argv to xFilter */
>         unsigned char omit;      /* Do not code a test for this constraint */
>       } *aConstraintUsage;
>       int idxNum;                /* Number used to identify the index */
>       char *idxStr;              /* String, possibly obtained from sqlite3_malloc */
>       int needToFreeIdxStr;      /* Free idxStr using sqlite3_free() if true */
>       int orderByConsumed;       /* True if output is already ordered */
>       double estimatedCost;      /* Estimated cost of using this index */
>     };

The sqlite3_index_info structure and its substructures is used as part of the virtual table interface to pass information into and receive the reply from the xBestIndex method of a virtual table module. The fields under **Inputs** are the inputs to xBestIndex and are read-only. xBestIndex inserts its results into the **Outputs** fields.

The aConstraint[] array records WHERE clause constraints of the form:

> column OP expr

where OP is =, <, <=, >, or >=. The particular operator is stored in aConstraint[].op using one of the SQLITE_INDEX_CONSTRAINT_ values. The index of the column is stored in aConstraint[].iColumn. aConstraint[].usable is TRUE if the expr on the right-hand side can be evaluated (and thus the constraint is usable) and false if it cannot.

The optimizer automatically inverts terms of the form "expr OP column" and makes other simplifications to the WHERE clause in an attempt to get as many WHERE clause terms into the form shown above as possible. The aConstraint[] array only reports WHERE clause terms that are relevant to the particular virtual table being queried.

Information about the ORDER BY clause is stored in aOrderBy[]. Each term of aOrderBy records a column of the ORDER BY clause.

The xBestIndex method must fill aConstraintUsage[] with information about what parameters to pass to xFilter. If argvIndex>0 then the right-hand side of the corresponding aConstraint[] is evaluated and becomes the argvIndex-th entry in argv. If aConstraintUsage[].omit is true, then the constraint is assumed to be fully handled by the virtual table and is not checked again by SQLite.

The idxNum and idxPtr values are recorded and passed into the xFilter method. sqlite3_free() is used to free idxPtr if and only if needToFreeIdxPtr is true.

The orderByConsumed means that output from xFilter/xNext will occur in the correct order to satisfy the ORDER BY clause so that no separate sorting step is required.

The estimatedCost value is an estimate of the cost of doing the particular lookup. A full scan of a table with N entries should have a cost of N. A binary search of a table of N entries should have a cost of approximately log(N).

See also lists of Objects, Constants, and Functions.

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