[ImmutableDict](https://github.com/pallets/flask/blob/master/src/flask/app.py)

is a class within the flask.app module of the [Flask](/flask.html)

framework that is actually imported from the Werkzeug

[datastructures module](https://github.com/pallets/werkzeug/blob/master/src/werkzeug/datastructures.py).

The ImmutableDict class wraps a

[standard Python dictionary](https://docs.python.org/3/tutorial/datastructures.html#dictionaries)

so that values cannot be modified after initially being set.

[AppGroup](https://github.com/pallets/flask/blob/master/src/flask/cli.py)

is a class within the flask.cli module of the Flask project. It

works like the

[Click Group](https://click.palletsprojects.com/en/7.x/commands/)

class and automatically wraps the functions using

[with_appcontext](/flask-cli-with-appcontext-examples.html).

is a subclass of [AppGroup](/flask-cli-appgroup-examples.html) that provides

for loading more commands from a configured Flask app. Generally, only

advanced use cases will need to use this class.

meta: Python example code that shows how to use the mssql callable from the sqlalchemy.dialects module of the SQLAlchemy project.

meta: Python example code that shows how to use the pymysql callable from the sqlalchemy.dialects.mysql module of the SQLAlchemy project.

meta: Python example code that shows how to use the mysql callable from the sqlalchemy.dialects module of the SQLAlchemy project.

meta: Python example code that shows how to use the oracle callable from the sqlalchemy.dialects module of the SQLAlchemy project.

meta: Python example code that shows how to use the ARRAY constant from the sqlalchemy.dialects.postgresql module of the SQLAlchemy project.

title: sqlalchemy.dialects.postgresql.base PGCompiler Example Code

category: page

slug: sqlalchemy-dialects-postgresql-base-pgcompiler-examples

sortorder: 500031015

toc: False

sidebartitle: sqlalchemy.dialects.postgresql.base PGCompiler

meta: Example code for understanding how to use the PGCompiler class from the sqlalchemy.dialects.postgresql.base module of the SQLAlchemy project.

PGCompiler is a class within the sqlalchemy.dialects.postgresql.base module of the SQLAlchemy project.

[sqlalchemy-clickhouse](https://github.com/cloudflare/sqlalchemy-clickhouse)

is a [SQLAlchemy Dialect](https://docs.sqlalchemy.org/en/13/dialects/)

for communicating with the open source [ClickHouse](https://clickhouse.tech/)

database management system. ClickHouse is column-oriented and therefore

better for some use cases and worse for others compared to a traditional

[relational database](/databases.html).

The code for this project is open sourced under the

[MIT license](https://github.com/cloudflare/sqlalchemy-clickhouse/blob/master/LICENSE.txt)

while ClickHouse is provided as open source under the

[Apache License 2.0](https://github.com/ClickHouse/ClickHouse/blob/master/LICENSE).

[**sqlalchemy-clickhouse / base.py**](https://github.com/cloudflare/sqlalchemy-clickhouse/blob/master/././base.py)

import re

import sqlalchemy.types as sqltypes

from sqlalchemy import exc as sa_exc

from sqlalchemy import util as sa_util

from sqlalchemy.engine import default, reflection

from sqlalchemy.sql import compiler, expression

from sqlalchemy.sql.elements import quoted_name

~~from sqlalchemy.dialects.postgresql.base import PGCompiler, PGIdentifierPreparer

from sqlalchemy.types import (

CHAR, DATE, DATETIME, INTEGER, SMALLINT, BIGINT, DECIMAL, TIME,

TIMESTAMP, VARCHAR, BINARY, BOOLEAN, FLOAT, REAL)

VERSION = (0, 1, 0, None)

colspecs = {}

class ARRAY(sqltypes.TypeEngine):

__visit_name__ = 'ARRAY'

ischema_names = {

'Int64': INTEGER,

'Int32': INTEGER,

'Int16': INTEGER,

'Int8': INTEGER,

'UInt64': INTEGER,

'UInt32': INTEGER,

'UInt16': INTEGER,

'UInt8': INTEGER,

'Date': DATE,

'DateTime': DATETIME,

'Float64': FLOAT,

'Float32': FLOAT,

'String': VARCHAR,

'FixedString': VARCHAR,

'Enum': VARCHAR,

'Enum8': VARCHAR,

'Enum16': VARCHAR,

'Array': ARRAY,

}

class ClickHouseIdentifierPreparer(PGIdentifierPreparer):

def quote_identifier(self, value):

return self._escape_identifier(value)

def quote(self, ident, force=None):

if self._requires_quotes(ident):

return '"{}"'.format(ident)

return ident

~~class ClickHouseCompiler(PGCompiler):

def visit_count_func(self, fn, **kw):

return 'count{0}'.format(self.process(fn.clause_expr, **kw))

def visit_random_func(self, fn, **kw):

return 'rand()'

def visit_now_func(self, fn, **kw):

return 'now()'

def visit_current_date_func(self, fn, **kw):

return 'today()'

def visit_true(self, element, **kw):

return '1'

def visit_false(self, element, **kw):

return '0'

def visit_cast(self, cast, **kwargs):

if self.dialect.supports_cast:

return super(ClickHouseCompiler, self).visit_cast(cast, **kwargs)

else:

return self.process(cast.clause, **kwargs)