.Dd December 19, 2018
.Dt SQLITE3_INDEX_INFO 3
.Os
.Sh NAME
.Nm sqlite3_index_info
.Nd Virtual Table Indexing Information
.Sh SYNOPSIS
.Vt struct sqlite3_index_info ;
.Sh DESCRIPTION
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.
.Pp
The aConstraint[] array records WHERE clause constraints of the form:
.Bd -ragged
column OP expr
.Ed
.Pp
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.
.Pp
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.
.Pp
Information about the ORDER BY clause is stored in aOrderBy[].
Each term of aOrderBy records a column of the ORDER BY clause.
.Pp
The colUsed field indicates which columns of the virtual table may
be required by the current scan.
Virtual table columns are numbered from zero in the order in which
they appear within the CREATE TABLE statement passed to sqlite3_declare_vtab().
For the first 63 columns (columns 0-62), the corresponding bit is set
within the colUsed mask if the column may be required by SQLite.
If the table has at least 64 columns and any column to the right of
the first 63 is required, then bit 63 of colUsed is also set.
In other words, column iCol may be required if the expression (colUsed
& ((sqlite3_uint64)1 << (iCol>=63 ? 63 : iCol))) evaluates to non-zero.
.Pp
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.
.Pp
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.
.Pp
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.
.Pp
The estimatedCost value is an estimate of the cost of a particular
strategy.
A cost of N indicates that the cost of the strategy is similar to a
linear scan of an SQLite table with N rows.
A cost of log(N) indicates that the expense of the operation is similar
to that of a binary search on a unique indexed field of an SQLite table
with N rows.
.Pp
The estimatedRows value is an estimate of the number of rows that will
be returned by the strategy.
.Pp
The xBestIndex method may optionally populate the idxFlags field with
a mask of SQLITE_INDEX_SCAN_* flags.
Currently there is only one such flag - SQLITE_INDEX_SCAN_UNIQUE.
If the xBestIndex method sets this flag, SQLite assumes that the strategy
may visit at most one row.
.Pp
Additionally, if xBestIndex sets the SQLITE_INDEX_SCAN_UNIQUE flag,
then SQLite also assumes that if a call to the xUpdate() method is
made as part of the same statement to delete or update a virtual table
row and the implementation returns SQLITE_CONSTRAINT, then there is
no need to rollback any database changes.
In other words, if the xUpdate() returns SQLITE_CONSTRAINT, the database
contents must be exactly as they were before xUpdate was called.
By contrast, if SQLITE_INDEX_SCAN_UNIQUE is not set and xUpdate returns
SQLITE_CONSTRAINT, any database changes made by the xUpdate method
are automatically rolled back by SQLite.
.Pp
IMPORTANT: The estimatedRows field was added to the sqlite3_index_info
structure for SQLite version 3.8.2 (dateof:3.8.2).
If a virtual table extension is used with an SQLite version earlier
than 3.8.2, the results of attempting to read or write the estimatedRows
field are undefined (but are likely to included crashing the application).
The estimatedRows field should therefore only be used if sqlite3_libversion_number()
returns a value greater than or equal to 3008002.
Similarly, the idxFlags field was added for version 3.9.0
(dateof:3.9.0).
It may therefore only be used if sqlite3_libversion_number() returns
a value greater than or equal to 3009000.
.Sh SEE ALSO
.Xr sqlite3_malloc 3 ,
.Xr sqlite3_version 3 ,
.Xr SQLITE_INDEX_CONSTRAINT_EQ 3 ,
.Xr sqlite3_module 3