Geografic-Information-System/lib/python/temporal/core.py

"""!@package grass.temporal

@brief GRASS Python scripting module (temporal GIS functions)

Temporal GIS core functions to be used in library modules and scripts.

This module provides the functionality to create the temporal
SQL database and to establish a connection to the database.

Usage:

@code

>>> import grass.temporal as tgis
>>> # Create the temporal database
>>> tgis.init()
>>> # Establish a database connection
>>> dbif, connected = tgis.init_dbif(None)
>>> dbif.connect()
>>> # Execute a SQL statement
>>> dbif.execute_transaction("SELECT datetime(0, 'unixepoch', 'localtime');")
>>> # Mogrify an SQL statement
>>> dbif.mogrify_sql_statement(["SELECT name from raster_base where name = ?",
... ("precipitation",)])
"SELECT name from raster_base where name = 'precipitation'"
>>> dbif.close()

@endcode

(C) 2011-2013 by the GRASS Development Team
This program is free software under the GNU General Public
License (>=v2). Read the file COPYING that comes with GRASS
for details.

@author Soeren Gebbert
"""
import sys, traceback
import os
import locale
# i18N
import gettext
gettext.install('grasslibs', os.path.join(os.getenv("GISBASE"), 'locale'))

import grass.script.core as core
from datetime import datetime
from c_libraries_interface import *
# Import all supported database backends
# Ignore import errors since they are checked later
try:
    import sqlite3
except ImportError:
    pass
# Postgresql is optional, existence is checked when needed
try:
    import psycopg2
    import psycopg2.extras
except:
    pass

import atexit

###############################################################################

# Profiling function provided by the temporal framework
def profile_function(func):
    do_profiling = os.getenv("GRASS_TGIS_PROFILE")
    
    if do_profiling is not None:
        import cProfile, pstats, StringIO
        pr = cProfile.Profile()
        pr.enable()
        func()
        pr.disable()
        s = StringIO.StringIO()
        sortby = 'cumulative'
        ps = pstats.Stats(pr, stream=s).sort_stats(sortby)
        ps.print_stats()
        print s.getvalue()
    else:
        func()

# Global variable that defines the backend
# of the temporal GIS
# It can either be "sqlite" or "pg"
tgis_backend = None

# The version of the temporal framework
# this value must be an integer larger than 0
# Increase this value in case of backward incompatible changes in the TGIS API
tgis_version=2
# The version of the temporal database since framework and database version can differ
# this value must be an integer larger than 0
# Increase this value in case of backward incompatible changes
# temporal database SQL layout
tgis_db_version=2

# We need to access the current mapset quite often in the framework, so we make
# global variable that will be initiated when init() is called
current_mapset=None

###############################################################################

def get_current_mapset():
    """!Return the current mapset

       This is the fastest way to receive the current mapset.
       The current mapset is set by init() and stored in a global variable.
       This function provides access to this global variable.
    """
    global current_mapset
    return current_mapset

def _set_current_mapset(mapset=None):
    """!This functions set the global current mapset variable to
       the current mapset by calling g.gisenv.

       @param mapset The current mapset, g.gisenv will be called
                     if this variable is set to None
    """
    global current_mapset

    if mapset == None:
        mapset = core.gisenv()["MAPSET"]

    current_mapset = mapset

###############################################################################

# If this global variable is set True, then maps can only be registered in space time datasets
# with the same mapset. In addition, only maps in the current mapset can be inserted, updated or deleted from
# the temporal database. 
# Overwrite this global variable by: g.gisenv set="TGIS_DISABLE_MAPSET_CHECK=True"
# ATTENTION: Be aware to face corrupted temporal database in case this global variable is set to False.
#            This feature is highly experimental and violates the grass permission guidance.
enable_mapset_check = True
# If this global variable is set True, the timestamps of maps will be written as textfiles
# for each map that will be inserted or updated in the temporal database using the C-library 
# timestamp interface.
# Overwrite this global variable by: g.gisenv set="TGIS_DISABLE_TIMESTAMP_WRITE=True"
# ATTENTION: Be aware to face corrupted temporal database in case this global variable is set to False.
#            This feature is highly experimental and violates the grass permission guidance.
enable_timestamp_write = True

def get_enable_mapset_check():
    """!Return True if the mapsets should be checked while insert, updatem delete requests
       and space time dataset registration.
       
       If this global variable is set True, then maps can only be registered in space time datasets
       with the same mapset. In addition, only maps in the current mapset can be inserted, updated or deleted from
       the temporal database. 
       Overwrite this global variable by: g.gisenv set="TGIS_DISABLE_MAPSET_CHECK=True"
       
       ATTENTION: Be aware to face corrupted temporal database in case this global variable is set to False.
                  This feature is highly experimental and violates the grass permission guidance.
    """
    global enable_mapset_check
    return enable_mapset_check
    
def get_enable_timestamp_write():
    """!Return True if the map timestamps should be written to the spatial database metadata as well.
    
       If this global variable is set True, the timestamps of maps will be written as textfiles
       for each map that will be inserted or updated in the temporal database using the C-library 
       timestamp interface.
       Overwrite this global variable by: g.gisenv set="TGIS_DISABLE_TIMESTAMP_WRITE=True"
       
       ATTENTION: Be aware that C-libraries can not access timestamp informations if they are not
                  written as spatial database metadata, hence modules that make use of timestamps 
                  using the C-library interface will not work with maps that were created without 
                  writing the timestamps.
    """
    global enable_timestamp_write
    return enable_timestamp_write

###############################################################################

# The global variable that stores the PyGRASS Messenger object that
# provides a fast and exit safe interface to the C-library message functions
message_interface=None

def _init_tgis_message_interface(raise_on_error=False):
    """!Initiate the global mesage interface

       @param raise_on_error If True raise a FatalError exception in case of a fatal error,
                             call sys.exit(1) otherwise
    """
    global message_interface
    from grass.pygrass import messages
    message_interface = messages.Messenger(raise_on_error)

def get_tgis_message_interface():
    """!Return the temporal GIS message interface which is of type
       grass.pyhrass.message.Messenger()

       Use this message interface to print messages to stdout using the
       GRASS C-library messaging system.
    """
    global message_interface
    return message_interface

###############################################################################

# The global variable that stores the C-library interface object that
# provides a fast and exit safe interface to the C-library libgis,
# libraster, libraster3d and libvector functions
c_library_interface=None

def _init_tgis_c_library_interface():
    """!Set the global C-library interface variable that
       provides a fast and exit safe interface to the C-library libgis,
       libraster, libraster3d and libvector functions
    """
    global c_library_interface
    c_library_interface = CLibrariesInterface()

def get_tgis_c_library_interface():
    """!Return the C-library interface that
       provides a fast and exit safe interface to the C-library libgis,
       libraster, libraster3d and libvector functions
    """
    global c_library_interface
    return c_library_interface

###############################################################################

def get_tgis_version():
    """!Get the verion number of the temporal framework
       @return The version number of the temporal framework as string
    """
    global tgis_version
    return tgis_version

###############################################################################

def get_tgis_db_version():
    """!Get the verion number of the temporal framework
       @return The version number of the temporal framework as string
    """
    global tgis_db_version
    return tgis_db_version

###############################################################################

def get_tgis_metadata(dbif=None):
    """!Return the tgis metadata table as a list of rows (dicts)
               or None if not present

       @param dbif The database interface to be used
       @return The selected rows with key/value comumns or None
    """

    dbif, connected = init_dbif(dbif)

    # Select metadata if the table is present
    try:
        statement = "SELECT * FROM tgis_metadata;\n"
        dbif.cursor.execute(statement)
        rows = dbif.cursor.fetchall()
    except:
        rows = None

    if connected:
        dbif.close()

    return rows

###############################################################################

def get_temporal_dbmi_init_string(kv=None, grassenv=None):
    """!Return the database initialization string

       @param kv dictionary generated by grass.script.parse_command("t.connect", flags="pg")
       @param grassenv Grass environemntal variables created by grass.script.gisenv()
    """

    global temporal_dbmi_init_string

    if kv == None:
        kv = core.parse_command("t.connect", flags="pg")
    if grassenv == None:
        grassenv = core.gisenv()

    global tgis_backend
    msgr = get_tgis_message_interface()

    if tgis_backend == "sqlite":
        # We substitute GRASS variables if they are located in the database string
        # This behavior is in conjunction with db.connect
        if "database" in kv:
            string = kv["database"]
            string = string.replace("$GISDBASE", grassenv["GISDBASE"])
            string = string.replace(
                "$LOCATION_NAME", grassenv["LOCATION_NAME"])
            string = string.replace("$MAPSET", grassenv["MAPSET"])
            temporal_dbmi_init_string = string
            return string
        else:
            msgr.fatal(_("Unable to initialize the temporal GIS DBMI "
                         "interface. Use t.connect to specify the driver "
                         "and the database string"))
    elif tgis_backend == "pg":
        if "database" in kv:
            string = kv["database"]
            temporal_dbmi_init_string = string
            return string
    else:
        msgr.fatal(_("Unable to initialize the temporal GIS DBMI "
                     "interface. Use t.connect to specify the driver "
                     "and the database string"))

###############################################################################

def get_sql_template_path():
    base = os.getenv("GISBASE")
    base_etc = os.path.join(base, "etc")
    return os.path.join(base_etc, "sql")

###############################################################################

def stop_subprocesses():
    """!Stop the messenger and C-interface subprocesses
    """
    global message_interface
    global c_library_interface
    if message_interface:
        message_interface.stop()
    if c_library_interface:
        c_library_interface.stop()

atexit.register(stop_subprocesses)

###############################################################################

def init():
    """!This function set the correct database backend from GRASS environmental variables
       and creates the grass location database structure for raster,
       vector and raster3d maps as well as for the space-time datasets strds,
       str3ds and stvds in case it does not exists.
       
       Several global variables are initiated and the messenger and C-library interface
       subprocesses are spawned.
       
       The following g.gisenv variables are checked:
        - TGIS_RAISE_ON_ERROR
        - TGIS_DISABLE_MAPSET_CHECK
        - TGIS_DISABLE_TIMESTAMP_WRITE
        
       The following environmental variables are checked:
        - GRASS_TGIS_PROFILE

        ATTENTION: This functions must be called before any spatio-temporal processing
                   can be started
    """
    # We need to set the correct database backend from the environment variables
    global tgis_backend
    global enable_mapset_check
    global enable_timestamp_write
    
    core.run_command("t.connect", flags="c")
    kv = core.parse_command("t.connect", flags="pg")
    grassenv = core.gisenv()
    raise_on_error = False

    # Check the g.gisenv variable TGIS_RAISE_ON_ERROR
    if grassenv.has_key("TGIS_RAISE_ON_ERROR"):
        if grassenv["TGIS_RAISE_ON_ERROR"] == "True" or grassenv["TGIS_RAISE_ON_ERROR"] == "1":
            raise_on_error = True
            
    # Set the global variable current_mapset for fast mapset access
    _set_current_mapset(grassenv["MAPSET"])
    # Start the GRASS message interface server
    _init_tgis_message_interface(raise_on_error)
    # Start the C-library interface server
    _init_tgis_c_library_interface()
    
    msgr = get_tgis_message_interface()
    msgr.debug(1, "Inititate the temporal database")
    
    if raise_on_error is True:
        msgr.warning("TGIS_RAISE_ON_ERROR is True")

    # Set the mapset check and the timestamp write
    if grassenv.has_key("TGIS_DISABLE_MAPSET_CHECK"):
        if grassenv["TGIS_DISABLE_MAPSET_CHECK"] == "True" or grassenv["TGIS_DISABLE_MAPSET_CHECK"] == "1":
            enable_mapset_check = False
            msgr.warning("TGIS_DISABLE_MAPSET_CHECK is True")

    if grassenv.has_key("TGIS_DISABLE_TIMESTAMP_WRITE"):
        if grassenv["TGIS_DISABLE_TIMESTAMP_WRITE"] == "True" or grassenv["TGIS_DISABLE_TIMESTAMP_WRITE"] == "1":
            enable_timestamp_write = False
            msgr.warning("TGIS_DISABLE_TIMESTAMP_WRITE is True")

    if "driver" in kv:
        if kv["driver"] == "sqlite":
            tgis_backend = kv["driver"]
            try:
                import sqlite3
            except ImportError:
                msgr.error("Unable to locate the sqlite SQL Python interface module sqlite3.")
                raise
            dbmi = sqlite3
        elif kv["driver"] == "pg":
            tgis_backend = kv["driver"]
            try:
                import psycopg2
            except ImportError:
                msgr.error("Unable to locate the Postgresql SQL Python interface module psycopg2.")
                raise
            dbmi = psycopg2
        else:
            msgr.fatal(_("Unable to initialize the temporal DBMI interface. Use "
                         "t.connect to specify the driver and the database string"))
            dbmi = sqlite3
    else:
        # Set the default sqlite3 connection in case nothing was defined
        core.run_command("t.connect", flags="d")

    db_exists = False
    database = get_temporal_dbmi_init_string(kv=kv, grassenv=grassenv)
    dbif = SQLDatabaseInterfaceConnection()

    # Check if the database already exists
    if tgis_backend == "sqlite":
        # Check path of the sqlite database
        if os.path.exists(database):
            dbif.connect()
            # Check for raster_base table
            dbif.cursor.execute("SELECT name FROM sqlite_master WHERE type='table' AND name='raster_base';")
            name = dbif.cursor.fetchone()
            if name and name[0] == "raster_base":
                db_exists = True
            dbif.close()

    elif tgis_backend == "pg":
        # Connect to database
        dbif.connect()
        # Check for raster_base table
        dbif.cursor.execute("SELECT EXISTS(SELECT * FROM information_schema.tables "
                   "WHERE table_name=%s)", ('raster_base',))
        if dbif.cursor.fetchone()[0]:
            db_exists = True

    if db_exists == True:
        
        # Check the version of the temporal database
        # This version works only with database of version 2
        dbif.close()
        dbif.connect()
        metadata = get_tgis_metadata(dbif)
        dbif.close()
        if metadata is None:
            msgr.fatal(_("Unable to receiving temporal database metadata. Your temporal database is not supported."))
        
        for entry in metadata:
            if "tgis_version" in entry and entry[1] != str(get_tgis_version()):
                msgr.fatal(_("Unsupported temporal database. Version mismatch.\n"
                "Supported temporal API version is: %(api)i")%({"api":get_tgis_version()}))
            if "tgis_db_version" in entry and entry[1] != str(get_tgis_db_version()):
                msgr.fatal(_("Unsupported temporal database. Version mismatch.\n"
                "Supported temporal database version is: %(tdb)i")%( {"tdb":get_tgis_db_version()}))
        return

    create_temporal_database(dbif, database)

###############################################################################

def create_temporal_database(dbif, database):
    """!This function will create the temporal database

       It will create all tables and triggers that are needed to run
       the temporal GIS

       @param dbif The database interface to be used
    """
    global tgis_backend
    global tgis_version
    global tgis_db_version

    template_path = get_sql_template_path()
    msgr = get_tgis_message_interface()

    # Read all SQL scripts and templates
    map_tables_template_sql = open(os.path.join(
        template_path, "map_tables_template.sql"), 'r').read()
    raster_metadata_sql = open(os.path.join(
        get_sql_template_path(), "raster_metadata_table.sql"), 'r').read()
    raster3d_metadata_sql = open(os.path.join(template_path,
                                              "raster3d_metadata_table.sql"),
                                              'r').read()
    vector_metadata_sql = open(os.path.join(template_path,
                                            "vector_metadata_table.sql"),
                                            'r').read()
    raster_views_sql = open(os.path.join(template_path, "raster_views.sql"),
                            'r').read()
    raster3d_views_sql = open(os.path.join(template_path,
                                           "raster3d_views.sql"), 'r').read()
    vector_views_sql = open(os.path.join(template_path, "vector_views.sql"),
                            'r').read()

    stds_tables_template_sql = open(os.path.join(template_path,
                                                 "stds_tables_template.sql"),
                                                 'r').read()
    strds_metadata_sql = open(os.path.join(template_path,
                                           "strds_metadata_table.sql"),
                                           'r').read()
    str3ds_metadata_sql = open(os.path.join(template_path,
                                            "str3ds_metadata_table.sql"),
                                            'r').read()
    stvds_metadata_sql = open(os.path.join(template_path,
                                           "stvds_metadata_table.sql"),
                                           'r').read()
    strds_views_sql = open(os.path.join(template_path, "strds_views.sql"),
                           'r').read()
    str3ds_views_sql = open(os.path.join(template_path, "str3ds_views.sql"),
                            'r').read()
    stvds_views_sql = open(os.path.join(template_path, "stvds_views.sql"),
                           'r').read()

    # Create the raster, raster3d and vector tables SQL statements
    raster_tables_sql = map_tables_template_sql.replace("GRASS_MAP", "raster")
    vector_tables_sql = map_tables_template_sql.replace("GRASS_MAP", "vector")
    raster3d_tables_sql = map_tables_template_sql.replace(
        "GRASS_MAP", "raster3d")

    # Create the space-time raster, raster3d and vector dataset tables
    # SQL statements
    strds_tables_sql = stds_tables_template_sql.replace("STDS", "strds")
    stvds_tables_sql = stds_tables_template_sql.replace("STDS", "stvds")
    str3ds_tables_sql = stds_tables_template_sql.replace("STDS", "str3ds")

    msgr.message(_("Create temporal database: %s" % (database)))

    if tgis_backend == "sqlite":
        # We need to create the sqlite3 database path if it does not exists
        tgis_dir = os.path.dirname(database)
        if not os.path.exists(tgis_dir):
            os.makedirs(tgis_dir)
        # Set up the trigger that takes care of
        # the correct deletion of entries across the different tables
        delete_trigger_sql = open(os.path.join(template_path,
                                               "sqlite3_delete_trigger.sql"),
                                               'r').read()
        indexes_sql = open(os.path.join(template_path, "sqlite3_indexes.sql"), 'r').read()
    else:
        # Set up the trigger that takes care of
        # the correct deletion of entries across the different tables
        delete_trigger_sql = open(os.path.join(template_path,
                                            "postgresql_delete_trigger.sql"),
                                            'r').read()
        indexes_sql = open(os.path.join(template_path, "postgresql_indexes.sql"), 'r').read()

    # Connect now to the database
    if not dbif.connected:
        dbif.connect()

    # Execute the SQL statements for sqlite
    # Create the global tables for the native grass datatypes
    dbif.execute_transaction(raster_tables_sql)
    dbif.execute_transaction(raster_metadata_sql)
    dbif.execute_transaction(raster_views_sql)
    dbif.execute_transaction(vector_tables_sql)
    dbif.execute_transaction(vector_metadata_sql)
    dbif.execute_transaction(vector_views_sql)
    dbif.execute_transaction(raster3d_tables_sql)
    dbif.execute_transaction(raster3d_metadata_sql)
    dbif.execute_transaction(raster3d_views_sql)
    # Create the tables for the new space-time datatypes
    dbif.execute_transaction(strds_tables_sql)
    dbif.execute_transaction(strds_metadata_sql)
    dbif.execute_transaction(strds_views_sql)
    dbif.execute_transaction(stvds_tables_sql)
    dbif.execute_transaction(stvds_metadata_sql)
    dbif.execute_transaction(stvds_views_sql)
    dbif.execute_transaction(str3ds_tables_sql)
    dbif.execute_transaction(str3ds_metadata_sql)
    dbif.execute_transaction(str3ds_views_sql)

    # The delete trigger
    dbif.execute_transaction(delete_trigger_sql)
    # The indexes
    dbif.execute_transaction(indexes_sql)

    # Create the tgis metadata table to store the database
    # initial configuration
    # The metadata table content
    metadata = {}
    metadata["tgis_version"] = tgis_version
    metadata["tgis_db_version"] = tgis_db_version
    metadata["creation_time"] = datetime.today()
    _create_tgis_metadata_table(metadata, dbif)

    dbif.close()

###############################################################################

def _create_tgis_metadata_table(content, dbif=None):
    """!Create the temporal gis metadata table which stores all metadata
       information about the temporal database.

       @param content The dictionary that stores the key:value metadata
                      that should be stored in the metadata table
       @param dbif The database interface to be used
    """
    dbif, connected = init_dbif(dbif)
    statement = "CREATE TABLE tgis_metadata (key VARCHAR NOT NULL, value VARCHAR);\n";
    dbif.execute_transaction(statement)

    for key in content.keys():
        statement = "INSERT INTO tgis_metadata (key, value) VALUES " + \
                     "(\'%s\' , \'%s\');\n"%(str(key), str(content[key]))
        dbif.execute_transaction(statement)

    if connected:
        dbif.close()

###############################################################################

class SQLDatabaseInterfaceConnection():
    """!This class represents the database interface connection
       and provides access to the chisen backend modules.

       The following DBMS are supported:
         - sqlite via the sqlite3 standard library
         - postgresql via psycopg2

    """
    def __init__(self):
        self.connected = False
        global tgis_backend
        if tgis_backend == "sqlite":
            self.dbmi = sqlite3
        else:
            self.dbmi = psycopg2

        self.msgr = get_tgis_message_interface()

    def rollback(self):
        """
            Roll back the last transaction. This must be called
            in case a new query should be performed after a db error.

            This is only relevant for postgresql database.
        """
        if self.dbmi.__name__ == "psycopg2":
            if self.connected:
                self.connection.rollback()

    def connect(self):
        """!Connect to the DBMI to execute SQL statements

           Supported backends are sqlite3 and postgresql
        """
        self.database = get_temporal_dbmi_init_string()
        if self.dbmi.__name__ == "sqlite3":
            self.connection = self.dbmi.connect(self.database,
                    detect_types = self.dbmi.PARSE_DECLTYPES | self.dbmi.PARSE_COLNAMES)
            self.connection.row_factory = self.dbmi.Row
            self.connection.isolation_level = None
            self.cursor = self.connection.cursor()
            self.cursor.execute("PRAGMA synchronous = OFF")
            self.cursor.execute("PRAGMA journal_mode = MEMORY")
        elif self.dbmi.__name__ == "psycopg2":
            self.connection = self.dbmi.connect(self.database)
            #self.connection.set_isolation_level(dbmi.extensions.ISOLATION_LEVEL_AUTOCOMMIT)
            self.cursor = self.connection.cursor(
                cursor_factory = self.dbmi.extras.DictCursor)
        self.connected = True

    def close(self):
        """!Close the DBMI connection"""
        #print "Close connection to",  self.database
        self.connection.commit()
        self.cursor.close()
        self.connected = False

    def mogrify_sql_statement(self, content):
        """!Return the SQL statement and arguments as executable SQL string

           @param content The content as tuple with two entries, the first
                           entry is the SQL statement with DBMI specific
                           place holder (?), the second entry is the argument
                           list that should substitue the place holder.

           Usage:

           @code

           >>> init()
           >>> dbif = SQLDatabaseInterfaceConnection()
           >>> dbif.mogrify_sql_statement(["SELECT ctime FROM raster_base WHERE id = ?",
           ... ["soil@PERMANENT",]])
           "SELECT ctime FROM raster_base WHERE id = 'soil@PERMANENT'"

           @endcode
        """
        sql = content[0]
        args = content[1]

        if self.dbmi.__name__ == "psycopg2":
            if len(args) == 0:
                return sql
            else:
                if self.connected:
                    try:
                        return self.cursor.mogrify(sql, args)
                    except:
                        print sql, args
                        raise
                else:
                    self.connect()
                    statement = self.cursor.mogrify(sql, args)
                    self.close()
                    return statement

        elif self.dbmi.__name__ == "sqlite3":
            if len(args) == 0:
                return sql
            else:
                # Unfortunately as sqlite does not support
                # the transformation of sql strings and qmarked or
                # named arguments we must make our hands dirty
                # and do it by ourself. :(
                # Doors are open for SQL injection because of the
                # limited python sqlite3 implementation!!!
                pos = 0
                count = 0
                maxcount = 100
                statement = sql

                while count < maxcount:
                    pos = statement.find("?", pos + 1)
                    if pos == -1:
                        break

                    if args[count] is None:
                        statement = "%sNULL%s" % (statement[0:pos],
                                                  statement[pos + 1:])
                    elif isinstance(args[count], (int, long)):
                        statement = "%s%d%s" % (statement[0:pos], args[count],
                                                statement[pos + 1:])
                    elif isinstance(args[count], float):
                        statement = "%s%f%s" % (statement[0:pos], args[count],
                                                statement[pos + 1:])
                    else:
                        # Default is a string, this works for datetime
                        # objects too
                        statement = "%s\'%s\'%s" % (statement[0:pos],
                                                    str(args[count]),
                                                    statement[pos + 1:])
                    count += 1

                return statement

    def check_table(self, table_name):
        """!Check if a table exists in the temporal database

           @param table_name The name of the table to be checked for existance
           @return True if the table exists, False otherwise
        """
        table_exists = False
        connected = False
        if not self.connected:
            self.connect()
            connected = True

        # Check if the database already exists
        if self.dbmi.__name__ == "sqlite3":

            self.cursor.execute("SELECT name FROM sqlite_master WHERE type='table' AND name='%s';"%table_name)
            name = self.cursor.fetchone()
            if name and name[0] == table_name:
                table_exists = True
        else:
            # Check for raster_base table
            self.cursor.execute("SELECT EXISTS(SELECT * FROM information_schema.tables "
                    "WHERE table_name=%s)", ('%s'%table_name,))
            if self.cursor.fetchone()[0]:
                table_exists = True

        if connected:
            self.close()

        return table_exists

    def execute_transaction(self, statement):
        """!Execute a transactional SQL statement

            The BEGIN and END TRANSACTION statements will be added automatically
            to the sql statement

            @param statement The executable SQL statement or SQL script
        """
        connected = False
        if not self.connected:
            self.connect()
            connected = True

        sql_script = ""
        sql_script += "BEGIN TRANSACTION;\n"
        sql_script += statement
        sql_script += "END TRANSACTION;"

        try:
            if self.dbmi.__name__ == "sqlite3":
                self.cursor.executescript(statement)
            else:
                self.cursor.execute(statement)
            self.connection.commit()
        except:
            if connected:
                self.close()
            self.msgr.error(_("Unable to execute transaction:\n %(sql)s" % {"sql":statement}))
            raise

        if connected:
            self.close()

###############################################################################

def init_dbif(dbif):
    """!This method checks if the database interface connection exists,
        if not a new one will be created, connected and True will be returned

        Usage code sample:
        @code

        dbif, connect = tgis.init_dbif(None)

        sql = dbif.mogrify_sql_statement(["SELECT * FROM raster_base WHERE ? = ?"],
                                               ["id", "soil@PERMANENT"])
        dbif.execute_transaction(sql)

        if connect:
            dbif.close()

        @endcode
    """
    if dbif is None:
        dbif = SQLDatabaseInterfaceConnection()
        dbif.connect()
        return dbif, True

    return dbif, False

###############################################################################

if __name__ == "__main__":
    import doctest
    doctest.testmod()