Benjamin Booth > SQLite-VirtualTable > SQLite::VirtualTable

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NAME ^

SQLite::VirtualTable - Create SQLite Virtual Table extensions in Perl.

SYNOPSIS ^

on Perl:

  package MyVirtualTable;
  use base 'SQLite::VirtualTable';

  sub CREATE {
    ...

and then from your preferred SQLite application or language, as for instance, the sqlite3 shell:

  $ sqlite3
  sqlite> .load perlvtab.so
  sqlite> CREATE VIRTUAL TABLE foo USING perl ("MyVirtualTable", foo, bar, ...);
  sqlite> SELECT * FROM foo WHERE col1 AND col1 > 34;
  ...

DESCRIPTION ^

Virtual tables are a new feature in SQLite (currently still only available from the development version on CVS) that allows you to create tables using custom backends to access (read and change) their contents instead of being stored in the database file.

The SQLite::VirtualTable module allows you to create these backends in Perl embbeding a perl interpreter as a SQLite extension.

Note that extensions written using this module can be used from any SQLite application and programming language (C, Java, PHP, Perl, etc.).

API ^

In order to provide a new backend, a Perl module derived from SQLite::VirtualTable has to be created and a set of methods defined.

These methods are just the Perl equivalents of the C callbacks defined on the SQLite Virtual Table specification available from http://www.sqlite.org/cvstrac/wiki?p=VirtualTableMethods.

To indicate failure they should die.

$class->CREATE($module, $dbname, $tablename, @args)
$class->CONNECT($module, $dbname, $tablename, @args)

These methods are called when the user enters the SQL CREATE VIRTUAL TABLE command or when the database containing the virtual table is opened.

They have to return a new object representing the virtual table.

@args contains the arguments included by the user on the SQL statement after the module name. They can be quoted and you would probably want to unquote them (see "unquote" in SQLite::VirtualTable::Util).

$vt->DECLARE_SQL()

This method is called just after the CREATE or CONNECT method and has to return the SQL statement used to declare the columns and types of the virtual table. For instance:

  sub DECLARE_SQL {
    my $self = shift;
    "CREATE TABLE $self->{name} (bar VARCHAR(10), doz INT)"
  }

The return value from this method is used when calling the C function sqlite3_declare_vtab() to register the virtual table.

$vt->DROP()

This method is called when the user runs the SQL DROP TABLE statement on the virtual table.

Note that the equivalent C callback is xDestroy but DESTROY was already used in Perl for other purposes.

$vt->DISCONNECT()

This method is called when the database is closed.

$vt->BEST_INDEX($constraints, $orderbys)

The documentation for the C equivalent of this callback is available from http://www.sqlite.org/cvstrac/wiki?p=VirtualTableBestIndexMethod.

The Perl method is called with two arguments representing the input part of the sqlite3_index_info C structure:

$constraints

is an array of hashes describing the possible constraints on the WHERE clause.

Every hash contains the entries column, operator and usable.

operator entries can take the values eq, gt, ge, lt, le and match.

This data structure is also used for output. The entries arg_index (note that it is not argv_index) and omit can be used to set the corresponding slots on the sqlite3_index_info C structure.

arg_index indexes start at 0 while on the C version of the callback they start at 1. An undef value or just not creating the entry indicates that the constraint is not going to be used on the filter.

$ordersby

is an array of hashes describing the possible ORDER BY clauses on the SQL statement.

On every hash the entries column and direction are defined.

direction entries take the values 1 or -1 for ascending or descending order respectively.

This method has to return the four values corresponging to idxNum, idxStr, orderByConsumed and estimatedCost on the sqlite3_index_info C structure.

$vt->OPEN

This method has to return an object representing a new cursor over the virtual table.

$vt->CLOSE($cursor)

This method is called to release a cursor.

$vt->FILTER($cursor, $idxnum, $idxstr, @args)

This method is called to begin a search of a virtual table.

It has to initialize the cursor previously created on a OPEN call.

$idsnum and $idxstr are the values returned by any of the BEST_NODE method calls previously performed. @args are the arguments to the WHERE constraints.

$vt->NEXT($cursor)

This method is called to advance the cursor to the next row.

$vt->EOF($cursor)

This method has to return a true value when the rows from the cursor have been exhausted.

$vt->UPDATE($delete_rowid, $new_rowid, @values)

This method is called when INSERT, UPDATE and DELETE actions are carried over the virtual table.

See the docs for the equivalent C callback for the details.

$vt->BEGIN_TRANSACTION()
$vt->SYNC_TRANSACTION()
$vt->COMMIT_TRANSACTION()
$vt->ROLLBACK_TRANSACTION()

These methods are called on transaction related events.

The default implementations from SQLite::VirtualTable do nothing.

$vt->RENAME($name)

Notification that the table will be given a new name. If a false value is returned, the rename operation will be cancled.

Has a default implementation that returns always true.

INSTALLATION ^

Before compiling you will have to ensure that the development files for sqlite3 and the perl library are installed. For instance, in Debian (and derivates as Ubuntu), you will have to install libsqlite3-dev and libperl-dev.

Compile the module as usual:

  $ perl Makefile.PL
  $ make
  $ make test

Then, (maybe as root) install the Perl package:

  $ make install

And finally to make the SQLite dynamic extension (perlvtab.so, though the name extension can be different depending on your OS) available to any SQLite application, you may want to copy the library file blib/arch/auto/SQLite/VirtualTable/perlvtab.so to some place where your OS could find it, for instance /usr/local/lib.

Alternatively, you could use LD_LIBRARY_PATH to make your OS look for it at a different place. Read the documentation for your OS dynamic linker/loader for the details (ld.so(8) under Linux).

If your Perl virtual table backend packages are not installed on the common places where the perl interpreter searchs for modules by default, you would also need to set the PERL5LIB variable conveniently (see perlrun).

USAGE FROM SQLITE ^

Once, the dynamic extension has been installed, you can use it on your SQLite C applications with the sqlite3_load_extension() function.

Or from the sqlite3 shell as:

  sqlite3> .load perlvtab.so

Or from SQL as

  SELECT load_extension('perlvtab.so');

Note that for security reasons, loading of dynamic extension could be disabled on your SQLite application. Read the documentation about dynamic extension available from the SQLite wiki http://www.sqlite.org/cvstrac/wiki?p=LoadableExtensions for the details.

The SQL syntax to create virtual tables is:

  CREATE VIRTUAL TABLE table_name USING perl ("Perl::Backend", arg0, arg1, ...);

Where Perl::Backend is the name of the module implementing the desired virtual table functionality.

After the table has been created, it can be used as any regular table from SQL.

USAGE FROM DBD::SQLite ^

SQLite::VirtualTable can also be used within perl scripts using DBD::SQLite. Using SQLite::VirtualTable with DBD::SQLite requires using the alternate entry point function dbd_sqlite_init_vtab_extension(). Here is the syntax:

  $dbh->sqlite_enable_load_extension(1);
  $dbh->sqlite_load_extension('perlvtab.so','dbd_sqlite_init_vtab_extension');

SEE ALSO ^

SQLite website http://www.sqlite.org/, including docs http://www.sqlite.org/docs.html and wiki http://www.sqlite.org/cvstrac/wiki.

The virtual table specification is currently available from http://www.sqlite.org/cvstrac/wiki?p=VirtualTables. The specification for loadable modules is available from http://www.sqlite.org/cvstrac/wiki?p=LoadableExtensions.

The manual page for the sqlite3 aplication sqlite3(1).

For a sample backend, see the SQLite::VirtualTable::CSV module that adds support for CSV files.

BUGS ^

Method xFindFunction is not supported.

This is alpha software based on an experimental feature of SQLite, lots of bugs are likely to appear.

The API could change in the future (well, actually, it is expected to change!!!).

Only tested on Linux.

Send bugs, comments or any feedback directly to me by mail or use the bug tracking system at http://rt.perl.org.

AUTHOR ^

Salvador Fandiño (sfandino@yahoo.com).

COPYRIGHT AND LICENSE ^

Copyright (C) 2006, 2009 by Qindel Formacion y Servicios, S. L.

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.8.8 or, at your option, any later version of Perl 5 you may have available.

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