port index creation to python

Also switches to jinja-based preprocessing, which allows to
simplify the SQL files. Use 'if not exists' where possible
so that the step can be rerun to fix missing indexes.

nominatim/db/sql_preprocessor.py

@@ -25,6 +25,36 @@ def _get_tables(conn):
 
         return set((row[0] for row in list(cur)))
 
+
+def _setup_tablespace_sql(config):
+    """ Returns a dict with tablespace expressions for the different tablespace
+        kinds depending on whether a tablespace is configured or not.
+    """
+    out = {}
+    for subset in ('ADDRESS', 'SEARCH', 'AUX'):
+        for kind in ('DATA', 'INDEX'):
+            tspace = getattr(config, 'TABLESPACE_{}_{}'.format(subset, kind))
+            if tspace:
+                tspace = 'TABLESPACE "{}"'.format(tspace)
+            out['{}_{}'.format(subset.lower, kind.lower())] = tspace
+
+    return out
+
+
+def _setup_postgres_sql(conn):
+    """ Set up a dictionary with various Postgresql/Postgis SQL terms which
+        are dependent on the database version in use.
+    """
+    out = {}
+    pg_version = conn.server_version_tuple()
+    # CREATE INDEX IF NOT EXISTS was introduced in PG9.5.
+    # Note that you need to ignore failures on older versions when
+    # unsing this construct.
+    out['if_index_not_exists'] = ' IF NOT EXISTS ' if pg_version >= (9, 5, 0) else ''
+
+    return out
+
+
 class SQLPreprocessor: # pylint: disable=too-few-public-methods
     """ A environment for preprocessing SQL files from the
         lib-sql directory.
@@ -44,17 +74,11 @@ class SQLPreprocessor: # pylint: disable=too-few-public-methods
         db_info['partitions'] = _get_partitions(conn)
         db_info['tables'] = _get_tables(conn)
         db_info['reverse_only'] = 'search_name' not in db_info['tables']
-
-        db_info['tablespace'] = {}
-        for subset in ('ADDRESS', 'SEARCH', 'AUX'):
-            for kind in ('DATA', 'INDEX'):
-                tspace = getattr(config, 'TABLESPACE_{}_{}'.format(subset, kind))
-                if tspace:
-                    tspace = 'TABLESPACE "{}"'.format(tspace)
-                db_info['tablespace']['{}_{}'.format(subset.lower, kind.lower())] = tspace
+        db_info['tablespace'] = _setup_tablespace_sql(config)
 
         self.env.globals['config'] = config
         self.env.globals['db'] = db_info
+        self.env.globals['sql'] = _setup_postgres_sql(conn)
         self.env.globals['modulepath'] = config.DATABASE_MODULE_PATH or \
                                          str((config.project_dir / 'module').resolve())