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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.