.. Automatically generated by code2rst.py
   Edit src/vtable.c not this file!

.. currentmodule:: apsw

.. _virtualtables:

Virtual Tables
**************

`Virtual Tables <https://sqlite.org/vtab.html>`__ are a feature
introduced in SQLite 3.3.7. They let a developer provide an underlying
table implementations, while still presenting a normal SQL interface
to the user. The person writing SQL doesn't need to know or care that
some of the tables come from elsewhere.

Some examples of how you might use this:

* Translating to/from information stored in other formats (eg a csv/ini format file)

* Accessing the data remotely (eg you could make a table that backends into Amazon's API)

* Dynamic information (eg currently running processes, files and directories, objects in your program)

* Information that needs reformatting (eg if you have complex rules about how to convert bytes to/from Unicode
  in the dataset)

* Information that isn't relationally correct (eg if you have data that has ended up with duplicate "unique" keys
  with code that dynamically corrects it)

* There are other examples on the `SQLite page <https://sqlite.org/vtab.html>`__

.. tip::

  You'll find initial development a lot quicker by using
  :meth:`apsw.ext.make_virtual_module`.  To write your own
  you will need to understand `xBestIndex <https://www.sqlite.org/vtab.html#the_xbestindex_method>`__.

To write a virtual table, you need to have 3 types of object. A
:class:`module <VTModule>`, a :class:`virtual table <VTTable>` and a
:class:`cursor <VTCursor>`. These are documented below. You can also
read the `SQLite C method documentation <https://sqlite.org/vtab.html>`__.
At the C level, they are just one set of methods. At the Python/APSW level,
they are split over the 3 types of object. The leading **x** is
omitted in Python. You can return SQLite error codes (eg
*SQLITE_READONLY*) by raising the appropriate exceptions (eg
:exc:`ReadOnlyError`).  :meth:`exceptionfor` is a useful helper
function to do the mapping.

IndexInfo class
===============

.. class:: IndexInfo

  IndexInfo represents the `sqlite3_index_info
  <https://www.sqlite.org/c3ref/index_info.html>`__ and associated
  methods used in the :meth:`VTTable.BestIndexObject` method.  The
  structure values are not altered or made friendlier in any way.

  Naming is identical to the C structure rather than Pythonic.  You can
  access members directly while needing to use get/set methods for array
  members.

  You will get :exc:`ValueError` if you use the object outside of an
  BestIndex method.

  :meth:`apsw.ext.index_info_to_dict` provides a convenient
  representation of this object as a :class:`dict`.

.. attribute:: IndexInfo.colUsed
  :type: set[int]

  (Read-only) Columns used by the statement.  Note that a set is returned, not
  the underlying integer.

.. index:: sqlite3_vtab_distinct

.. attribute:: IndexInfo.distinct
  :type: int

  (Read-only) How the query planner would like output ordered

  Calls: `sqlite3_vtab_distinct <https://sqlite.org/c3ref/vtab_distinct.html>`__

.. attribute:: IndexInfo.estimatedCost
  :type: float

  Estimated cost of using this index

.. attribute:: IndexInfo.estimatedRows
  :type: int

  Estimated number of rows returned

.. method:: IndexInfo.get_aConstraintUsage_argvIndex(which: int) -> int

 Returns *argvIndex* for *aConstraintUsage[which]*

.. index:: sqlite3_vtab_in

.. method:: IndexInfo.get_aConstraintUsage_in(which: int) -> bool

 Returns True if the constraint is *in* - eg column in (3, 7, 9)

 Calls: `sqlite3_vtab_in <https://sqlite.org/c3ref/vtab_in.html>`__

.. method:: IndexInfo.get_aConstraintUsage_omit(which: int) -> bool

 Returns *omit* for *aConstraintUsage[which]*

.. index:: sqlite3_vtab_collation

.. method:: IndexInfo.get_aConstraint_collation(which: int) -> str

 Returns collation name for *aConstraint[which]*

 Calls: `sqlite3_vtab_collation <https://sqlite.org/c3ref/vtab_collation.html>`__

.. method:: IndexInfo.get_aConstraint_iColumn(which: int) -> int

 Returns *iColumn* for *aConstraint[which]*

.. method:: IndexInfo.get_aConstraint_op(which: int) -> int

 Returns *op* for *aConstraint[which]*

.. index:: sqlite3_vtab_rhs_value

.. method:: IndexInfo.get_aConstraint_rhs(which: int) -> SQLiteValue

 Returns right hand side value if known, else None.

 Calls: `sqlite3_vtab_rhs_value <https://sqlite.org/c3ref/vtab_rhs_value.html>`__

.. method:: IndexInfo.get_aConstraint_usable(which: int) -> bool

 Returns *usable* for *aConstraint[which]*

.. method:: IndexInfo.get_aOrderBy_desc(which: int) -> bool

 Returns *desc* for *aOrderBy[which]*

.. method:: IndexInfo.get_aOrderBy_iColumn(which: int) -> int

 Returns *iColumn* for *aOrderBy[which]*

.. attribute:: IndexInfo.idxFlags
  :type: int

  Mask of :attr:`SQLITE_INDEX_SCAN flags <apsw.mapping_virtual_table_scan_flags>`

.. attribute:: IndexInfo.idxNum
  :type: int

  Number used to identify the index

.. attribute:: IndexInfo.idxStr
  :type: Optional[str]

  Name used to identify the index

.. attribute:: IndexInfo.nConstraint
  :type: int

  (Read-only) Number of constraint entries

.. attribute:: IndexInfo.nOrderBy
  :type: int

  (Read-only) Number of order by  entries

.. attribute:: IndexInfo.orderByConsumed
  :type: bool

  True if index output is already ordered

.. method:: IndexInfo.set_aConstraintUsage_argvIndex(which: int, argvIndex: int) -> None

 Sets *argvIndex* for *aConstraintUsage[which]*

.. index:: sqlite3_vtab_in

.. method:: IndexInfo.set_aConstraintUsage_in(which: int, filter_all: bool) -> None

 If *which* is an *in* constraint, and *filter_all* is True then your :meth:`VTCursor.Filter`
 method will have all of the values at once.

 Calls: `sqlite3_vtab_in <https://sqlite.org/c3ref/vtab_in.html>`__

.. method:: IndexInfo.set_aConstraintUsage_omit(which: int, omit: bool) -> None

 Sets *omit* for *aConstraintUsage[which]*

VTModule class
==============

.. class:: VTModule

.. note::

  There is no actual *VTModule* class - it is shown this way for
  documentation convenience and is present as a `typing protocol
  <https://docs.python.org/3/library/typing.html#typing.Protocol>`__.
  Your module instance should implement all the methods documented here.

A module instance is used to create the virtual tables.  Once you have
a module object, you register it with a connection by calling
:meth:`Connection.createmodule`::

  # make an instance
  mymod=MyModuleClass()

  # register the vtable on connection con
  con.createmodule("modulename", mymod)

  # tell SQLite about the table
  con.execute("create VIRTUAL table tablename USING modulename('arg1', 2)")

The create step is to tell SQLite about the existence of the table.
Any number of tables referring to the same module can be made this
way.  Note the (optional) arguments which are passed to the module.

.. method:: VTModule.Connect(connection: Connection, modulename: str, databasename: str, tablename: str, *args: tuple[SQLiteValue, ...])  -> tuple[str, VTTable]

    The parameters and return are identical to
    :meth:`~VTModule.Create`.  This method is called
    when there are additional references to the table.  :meth:`~VTModule.Create` will be called the first time and
    :meth:`~VTModule.Connect` after that.

    The advise is to create caches, generated data and other
    heavyweight processing on :meth:`~VTModule.Create` calls and then
    find and reuse that on the subsequent :meth:`~VTModule.Connect`
    calls.

    The corresponding call is :meth:`VTTable.Disconnect`.  If you have a simple virtual table implementation, then just
    set :meth:`~VTModule.Connect` to be the same as :meth:`~VTModule.Create`::

      class MyModule:

           def Create(self, connection, modulename, databasename, tablename, *args):
               # do lots of hard work

           Connect=Create

.. method:: VTModule.Create(connection: Connection, modulename: str, databasename: str, tablename: str, *args: tuple[SQLiteValue, ...])  -> tuple[str, VTTable]

   Called when a table is first created on a :class:`connection
   <Connection>`.

   :param connection: An instance of :class:`Connection`
   :param modulename: The string name under which the module was :meth:`registered <Connection.createmodule>`
   :param databasename: The name of the database.  This will be ``main`` for directly opened files and the name specified in
           `ATTACH <https://sqlite.org/lang_attach.html>`_ statements.
   :param tablename: Name of the table the user wants to create.
   :param args: Any arguments that were specified in the `create virtual table <https://sqlite.org/lang_createvtab.html>`_ statement.

   :returns: A list of two items.  The first is a SQL `create table <https://sqlite.org/lang_createtable.html>`_ statement.  The
        columns are parsed so that SQLite knows what columns and declared types exist for the table.  The second item
        is an object that implements the :class:`table <VTTable>` methods.

   The corresponding call is :meth:`VTTable.Destroy`.

.. method:: VTModule.ShadowName(table_suffix: str) -> bool

  This method is called to check if
  *table_suffix* is a `shadow name
  <https://www.sqlite.org/vtab.html#the_xshadowname_method>`__

  The default implementation always returns *False*.

  If a virtual table is created using this module
  named :code:`example` and then a  real table is created
  named :code:`example_content`, this would be called with
  a *table_suffix* of :code:`content`

VTTable class
=============

.. class:: VTTable

  .. note::

    There is no actual *VTTable* class - it is shown this way for
    documentation convenience and is present as a `typing protocol
    <https://docs.python.org/3/library/typing.html#typing.Protocol>`__.
    Your table instance should implement the methods documented here.

  The :class:`VTTable` object contains knowledge of the indices, makes
  cursors and can perform transactions.

  .. _vtablestructure:

  A virtual table is structured as a series of rows, each of which has
  the same number of columns.  The value in a column must be one of the `5
  supported types <https://sqlite.org/datatype3.html>`_, but the
  type can be different between rows for the same column.  The virtual
  table routines identify the columns by number, starting at zero.

  Each row has a **unique** 64 bit integer `rowid
  <https://sqlite.org/autoinc.html>`_ with the :class:`Cursor
  <VTCursor>` routines operating on this number, as well as some of
  the :class:`Table <VTTable>` routines such as :meth:`UpdateChangeRow
  <VTTable.UpdateChangeRow>`.

  It is possible to not have a rowid - read more at `the SQLite
  site <https://www.sqlite.org/vtab.html#_without_rowid_virtual_tables_>`__

.. method:: VTTable.Begin() -> None

  This function is used as part of transactions.  You do not have to
  provide the method.

.. method:: VTTable.BestIndex(constraints: Sequence[tuple[int, int]], orderbys: Sequence[tuple[int, int]]) -> Any

  This is a complex method. To get going initially, just return
  *None* and you will be fine. You should also consider using
  :meth:`BestIndexObject` instead.

  Implementing this method reduces the number of rows scanned
  in your table to satisfy queries, but only if you have an
  index or index like mechanism available.

  .. note::

    The implementation of this method differs slightly from the
    `SQLite documentation
    <https://sqlite.org/vtab.html>`__
    for the C API. You are not passed "unusable" constraints. The
    argv/constraintarg positions are not off by one. In the C api, you
    have to return position 1 to get something passed to
    :meth:`VTCursor.Filter` in position 0. With the APSW
    implementation, you return position 0 to get Filter arg 0,
    position 1 to get Filter arg 1 etc.

  The purpose of this method is to ask if you have the ability to
  determine if a row meets certain constraints that doesn't involve
  visiting every row. An example constraint is ``price > 74.99``. In a
  traditional SQL database, queries with constraints can be speeded up
  `with indices <https://sqlite.org/lang_createindex.html>`_. If
  you return None, then SQLite will visit every row in your table and
  evaluate the constraint itself. Your index choice returned from
  BestIndex will also be passed to the :meth:`~VTCursor.Filter` method on your cursor
  object. Note that SQLite may call this method multiple times trying
  to find the most efficient way of answering a complex query.

  **constraints**

  You will be passed the constraints as a sequence of tuples containing two
  items. The first item is the column number and the second item is
  the operation.

     Example query: ``select * from foo where price > 74.99 and
     quantity<=10 and customer='Acme Widgets'``

     If customer is column 0, price column 2 and quantity column 5
     then the constraints will be::

       (2, apsw.SQLITE_INDEX_CONSTRAINT_GT),
       (5, apsw.SQLITE_INDEX_CONSTRAINT_LE),
       (0, apsw.SQLITE_INDEX_CONSTRAINT_EQ)

     Note that you do not get the value of the constraint (ie "Acme
     Widgets", 74.99 and 10 in this example).

  If you do have any suitable indices then you return a sequence the
  same length as constraints with the members mapping to the
  constraints in order. Each can be one of None, an integer or a tuple
  of an integer and a boolean.  Conceptually SQLite is giving you a
  list of constraints and you are returning a list of the same length
  describing how you could satisfy each one.

  Each list item returned corresponding to a constraint is one of:

     None
       This means you have no index for that constraint. SQLite
       will have to iterate over every row for it.

     integer
       This is the argument number for the constraintargs being passed
       into the :meth:`~VTCursor.Filter` function of your
       :class:`cursor <VTCursor>` (the values "Acme Widgets", 74.99
       and 10 in the example).

     (integer, boolean)
       By default SQLite will check what you return. For example if
       you said that you had an index on price and so would only
       return rows greater than 74.99, then SQLite will still
       check that each row you returned is greater than 74.99.
       If the boolean is True then SQLite will not double
       check, while False retains the default double checking.

  Example query: ``select * from foo where price > 74.99 and
  quantity<=10 and customer=='Acme Widgets'``.  customer is column 0,
  price column 2 and quantity column 5.  You can index on customer
  equality and price.

  +----------------------------------------+--------------------------------+
  | Constraints (in)                       | Constraints used (out)         |
  +========================================+================================+
  | ::                                     | ::                             |
  |                                        |                                |
  |  (2, apsw.SQLITE_INDEX_CONSTRAINT_GT), |     1,                         |
  |  (5, apsw.SQLITE_INDEX_CONSTRAINT_LE), |     None,                      |
  |  (0, apsw.SQLITE_INDEX_CONSTRAINT_EQ)  |     0                          |
  |                                        |                                |
  +----------------------------------------+--------------------------------+

  When your :class:`~VTCursor.Filter` method in the cursor is called,
  constraintarg[0] will be "Acme Widgets" (customer constraint value)
  and constraintarg[1] will be 74.99 (price constraint value). You can
  also return an index number (integer) and index string to use
  (SQLite attaches no significance to these values - they are passed
  as is to your :meth:`VTCursor.Filter` method as a way for the
  BestIndex method to let the :meth:`~VTCursor.Filter` method know
  which of your indices or similar mechanism to use.

  **orderbys**

  The second argument to BestIndex is a sequence of orderbys because
  the query requested the results in a certain order. If your data is
  already in that order then SQLite can give the results back as
  is. If not, then SQLite will have to sort the results first.

    Example query: ``select * from foo order by price desc, quantity asc``

    Price is column 2, quantity column 5 so orderbys will be::

      (2, True),  # True means descending, False is ascending
      (5, False)

  **Return**

  You should return up to 5 items. Items not present in the return have a default value.

  0: constraints used (default None)
    This must either be None or a sequence the same length as
    constraints passed in. Each item should be as specified above
    saying if that constraint is used, and if so which constraintarg
    to make the value be in your :meth:`VTCursor.Filter` function.

  1: index number (default zero)
    This value is passed as is to :meth:`VTCursor.Filter`

  2: index string (default None)
    This value is passed as is to :meth:`VTCursor.Filter`

  3: orderby consumed (default False)
    Return True if your output will be in exactly the same order as the orderbys passed in

  4: estimated cost (default a huge number)
    Approximately how many disk operations are needed to provide the
    results. SQLite uses the cost to optimise queries. For example if
    the query includes *A or B* and A has 2,000 operations and B has 100
    then it is best to evaluate B before A.

  **A complete example**

  Query is ``select * from foo where price>74.99 and quantity<=10 and
  customer=="Acme Widgets" order by price desc, quantity asc``.
  Customer is column 0, price column 2 and quantity column 5. You can
  index on customer equality and price.

  ::

    BestIndex(constraints, orderbys)

    constraints= ( (2, apsw.SQLITE_INDEX_CONSTRAINT_GT),
                   (5, apsw.SQLITE_INDEX_CONSTRAINT_LE),
                   (0, apsw.SQLITE_INDEX_CONSTRAINT_EQ)  )

    orderbys= ( (2, True), (5, False) )

    # You return

    ( (1, None, 0),   # constraints used
      27,             # index number
      "idx_pr_cust",  # index name
      False,          # results are not in orderbys order
      1000            # about 1000 disk operations to access index
    )

    # Your Cursor.Filter method will be called with:

    27,              # index number you returned
    "idx_pr_cust",   # index name you returned
    "Acme Widgets",  # constraintarg[0] - customer
    74.99            # constraintarg[1] - price

.. method:: VTTable.BestIndexObject(index_info: IndexInfo) -> bool

  This method is called instead of :meth:`BestIndex` if
  *use_bestindex_object* was *True* in the call to
  :meth:`Connection.createmodule`.

  Use the :class:`IndexInfo` to tell SQLite about your indexes, and
  extract other information.

  Return *True* to indicate all is well.  If you return *False* or there is an error,
  then `SQLITE_CONSTRAINT
  <https://www.sqlite.org/vtab.html#return_value>`__ is returned to
  SQLite.

.. method:: VTTable.Commit() -> None

  This function is used as part of transactions.  You do not have to
  provide the method.

.. method:: VTTable.Destroy() -> None

  The opposite of :meth:`VTModule.Create`.  This method is called when
  the table is no longer used.  Note that you must always release
  resources even if you intend to return an error, as it will not be
  called again on error.  SQLite may also leak memory
  if you return an error.

.. method:: VTTable.Disconnect() -> None

  The opposite of :meth:`VTModule.Connect`.  This method is called when
  a reference to a virtual table is no longer used, but :meth:`VTTable.Destroy` will
  be called when the table is no longer used.

.. method:: VTTable.FindFunction(name: str, nargs: int) -> None |  Callable | tuple[int, Callable]

  Called to find if the virtual table has its own implementation of a
  particular scalar function. You do not have to provide this method.

  :param name: The function name
  :param nargs: How many arguments the function takes

  Return *None* if you don't have the function.  Zero is then returned to SQLite.

  Return a callable if you have one.  One is then returned to SQLite with the function.

  Return a sequence of int, callable.  The int is returned to SQLite with the function.
  This is useful for *SQLITE_INDEX_CONSTRAINT_FUNCTION* returns.

  It isn't possible to tell SQLite about exceptions in this function, so an
  :ref:`unraisable exception <unraisable>` is used.

  .. seealso::

    * :meth:`Connection.overloadfunction`
    * `FindFunction documentation <https://www.sqlite.org/vtab.html#xfindfunction>`__

.. method:: VTTable.Open() -> VTCursor

  Returns a :class:`cursor <VTCursor>` object.

.. method:: VTTable.Release(level: int) -> None

  Release nested transactions back to *level*.

  If you do not provide this method then the call succeeds (matching
  SQLite behaviour when no callback is provided).

.. method:: VTTable.Rename(newname: str) -> None

  Notification that the table will be given a new name. If you return
  without raising an exception, then SQLite renames the table (you
  don't have to do anything). If you raise an exception then the
  renaming is prevented.  You do not have to provide this method.

.. method:: VTTable.Rollback() -> None

  This function is used as part of transactions.  You do not have to
  provide the method.

.. method:: VTTable.Savepoint(level: int) -> None

  Set nested transaction to *level*.

  If you do not provide this method then the call succeeds (matching
  SQLite behaviour when no callback is provided).

.. method:: VTTable.Sync() -> None

  This function is used as part of transactions.  You do not have to
  provide the method.

.. method:: VTTable.UpdateChangeRow(row: int, newrowid: int, fields: tuple[SQLiteValue, ...]) -> None

  Change an existing row.  You may also need to change the rowid - for example if the query was
  ``UPDATE table SET rowid=rowid+100 WHERE ...``

  :param row: The existing 64 bit integer rowid
  :param newrowid: If not the same as *row* then also change the rowid to this.
  :param fields: A tuple of values the same length and order as columns in your table

.. method:: VTTable.UpdateDeleteRow(rowid: int) -> None

  Delete the row with the specified *rowid*.

  :param rowid: 64 bit integer

.. method:: VTTable.UpdateInsertRow(rowid: Optional[int], fields: tuple[SQLiteValue, ...])  -> Optional[int]

  Insert a row with the specified *rowid*.

  :param rowid: *None* if you should choose the rowid yourself, else a 64 bit integer
  :param fields: A tuple of values the same length and order as columns in your table

  :returns: If *rowid* was *None* then return the id you assigned
    to the row.  If *rowid* was not *None* then the return value
    is ignored.

VTCursor class
==============

.. class:: VTCursor

.. note::

  There is no actual *VTCursor* class - it is shown this way for
  documentation convenience and is present as a `typing protocol
  <https://docs.python.org/3/library/typing.html#typing.Protocol>`__.
  Your cursor instance should implement all the methods documented
  here.

The :class:`VTCursor` object is used for iterating over a table.
There may be many cursors simultaneously so each one needs to keep
track of where      :ref:`Virtual table structure <vtablestructure>`
it is.

.. seealso::

     :ref:`Virtual table structure <vtablestructure>`

.. method:: VTCursor.Close() -> None

  This is the destructor for the cursor. Note that you must
  cleanup. The method will not be called again if you raise an
  exception.

.. method:: VTCursor.Column(number: int) -> SQLiteValue

  Requests the value of the specified column *number* of the current
  row.  If *number* is -1 then return the rowid.

  :returns: Must be one one of the :ref:`5
    supported types <types>`

.. index:: sqlite3_vtab_nochange

.. method:: VTCursor.ColumnNoChange(number: int) -> SQLiteValue

  :meth:`VTTable.UpdateChangeRow` is going to be called which includes
  values for all columns.  However this column is not going to be changed
  in that update.

  If you return :attr:`apsw.no_change` then :meth:`VTTable.UpdateChangeRow`
  will have :attr:`apsw.no_change` for this column.  If you return
  anything else then it will have that value - as though :meth:`VTCursor.Column`
  had been called.

  This method will only be called if *use_no_change* was *True* in the
  call to :meth:`Connection.createmodule`.

  Calls: `sqlite3_vtab_nochange <https://sqlite.org/c3ref/vtab_nochange.html>`__

.. method:: VTCursor.Eof() -> bool

  Called to ask if we are at the end of the table. It is called after each call to Filter and Next.

  :returns: False if the cursor is at a valid row of data, else True

  .. note::

    This method can only return True or False to SQLite.  If you have
    an exception in the method or provide a non-boolean return then
    True (no more data) will be returned to SQLite.

.. index:: sqlite3_vtab_in_first, sqlite3_vtab_in_next

.. method:: VTCursor.Filter(indexnum: int, indexname: str, constraintargs: Optional[tuple]) -> None

  This method is always called first to initialize an iteration to the
  first row of the table. The arguments come from the
  :meth:`~VTTable.BestIndex` or :meth:`~VTTable.BestIndexObject`
  with constraintargs being a tuple of the constraints you
  requested. If you always return None in BestIndex then indexnum will
  be zero, indexstring will be None and constraintargs will be empty).

  If you had an *in* constraint and set :meth:`IndexInfo.set_aConstraintUsage_in`
  then that value will be a :class:`set`.

  Calls:
    * `sqlite3_vtab_in_first <https://sqlite.org/c3ref/vtab_in_first.html>`__
    * `sqlite3_vtab_in_next <https://sqlite.org/c3ref/vtab_in_first.html>`__

.. method:: VTCursor.Next() -> None

  Move the cursor to the next row.  Do not have an exception if there
  is no next row.  Instead return False when :meth:`~VTCursor.Eof` is
  subsequently called.

  If you said you had indices in your :meth:`VTTable.BestIndex`
  return, and they were selected for use as provided in the parameters
  to :meth:`~VTCursor.Filter` then you should move to the next
  appropriate indexed and constrained row.

.. method:: VTCursor.Rowid() -> int

  Return the current rowid.

Troubleshooting virtual tables
==============================

A big help is using the local variables recipe as described in
:ref:`augmented stack traces <augmentedstacktraces>` which will give
you more details in errors, and shows an example with the complex
:meth:`~VTTable.BestIndex` function.

You may also find errors compounding. For
example if you have an error in the Filter method of a cursor, SQLite
then closes the cursor. If you also return an error in the Close
method then the first error may mask the second or vice versa.

.. note::

   SQLite may ignore responses from your methods if they don't make
   sense. For example in BestIndex, if you set multiple arguments to
   have the same constraintargs position then your Filter won't
   receive any constraintargs at all.
