This document describes the xBestIndex method
of a {link: wiki?p=VirtualTables virtual table}
implementation.
If you have not already read about VirtualTables
you should do so before reading this page.
You might also want to read about the
{link: wiki?p=VirtualTableMethods other virtual table methods}
prior to perusing the following text.

The xBestIndex method has a prototype like this:

  int (*xBestIndex)(sqlite3_vtab *pVTab, sqlite3_index_info*);

Most of the input and output to and from the xBestIndex
method occurs inside the sqlite3_index_info structure.
The sqlite3_index_info structure looks like this:

  struct sqlite3_index_info {
    /* Inputs */
    const int nConstraint;     /* Number of entries in aConstraint */
    const 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 */
    } *const aConstraint;      /* Table of WHERE clause constraints */
    const int nOrderBy;        /* Number of terms in the ORDER BY clause */
    const struct sqlite3_index_orderby {
       int iColumn;              /* Column number */
       unsigned char desc;       /* True for DESC.  False for ASC. */
    } *const 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 */
    } *const 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 */
  };

In addition, there are some defined constants:

  #define SQLITE_INDEX_CONSTRAINT_EQ    2
  #define SQLITE_INDEX_CONSTRAINT_GT    4
  #define SQLITE_INDEX_CONSTRAINT_LE    8
  #define SQLITE_INDEX_CONSTRAINT_LT    16
  #define SQLITE_INDEX_CONSTRAINT_GE    32
  #define SQLITE_INDEX_CONSTRAINT_MATCH 64

The sqlite3_index_info and the constants shown above
are all defined in the sqlite3.h header file.