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PostgreSQL's Foreign Data Wrapper For ClickHouse
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README.md

clickhousedb_fdw - ClickHouse Foreign Data Wrapper for PostgreSQL

The clickhousedb_fdw is open-source. It is a Foreign Data Wrapper (FDW) for one of the fastest column store databases; "Clickhouse". This FDW allows you to SELECT from, and INSERT into, a ClickHouse database from within a PostgreSQL v11 server.

The FDW supports advanced features like aggregate pushdown and joins pushdown. These significantly improve performance by utilizing the remote server’s resources for these resource intensive operations.

1. Supported PostgreSQL Versions.

PostgreSQL Version 11 1.

2. Installation
Prerequisite

The clickhousedb_fdw uses an ODBC driver provided by ClickHouse2. This driver can be downloaded from the ClickHouse site3. This driver requires unixodbc4 which can be installed on any *nix based operating system using the platform’s package manager.

Installing clickhousedb_fdw

There are two ways to install the clickhousedb_fdw. The first is by downloading the clickhousedb_fdw source code and compiling it. The second option is to download the deb or rpm packages.

Download and compile

The latest release of clickhousedb_fdw can be downloaded from this GitHub page:

  https://github.com/Percona/clickhousedb_fdw/releases

or it can be downloaded using the git:

 git clone https://github.com/Percona/clickhousedb_fdw.git

After downloading the code, set the path for the PostgreSQL binary:

cd clickhousedb_fdw 
make USE_PGXS=1
	make USE_PGXS=1 install
Install using deb / rpm packages.

Sudo yum install clickhousedb_fdw Sudo apt-get install clickhousedb_fdw

Usage

You need to set up the sample database and tables in the ClickHouse database. Here we create a sample database name test_database and two sample tables tax_bills_nyc and tax_bills:

CREATE DATABASE test_database;

USE test_database;


CREATE TABLE tax_bills_nyc 
(
    bbl Int64,
    owner_name String,
    address String,
    tax_class String,
    tax_rate String,
    emv Float64,
    tbea Float64,
    bav Float64,
    tba String,
    property_tax String,
    condonumber String,
    condo String,
    insertion_date DateTime MATERIALIZED now() 
)
ENGINE = MergeTree PARTITION BY tax_class ORDER BY (owner_name)

Download the sample data from the taxbills.nyc website and put the data in the table:

curl -X GET 'http://taxbills.nyc/tax_bills_june15_bbls.csv' | clickhouse-client --input_format_allow_errors_num=10 --query="INSERT INTO test_database.tax_bills_nyc FORMAT CSV"

CREATE TABLE tax_bills
(
    bbl bigint, 
    owner_name text
)
ENGINE = MergeTree
PARTITION BY bbl
ORDER BY bbl;
PostgreSQL

Now the data is ready in the ClickHouse, the next step is to set up the PostgreSQL side. We need to create a ClickHouse foreign server, user mapping, and foreign tables.

CREATE SERVER clickhouse_svr FOREIGN DATA WRAPPER clickhousedb_fdw OPTIONS(dbname 'test_database', driver '/home/vagrant/percona/clickhousedb_fdw/lib/clickhouse-odbc/driver/libclickhouseodbc.so', host '127.0.0.1');

CREATE USER MAPPING FOR CURRENT_USER SERVER clickhouse_svr;
postgres=# CREATE FOREIGN TABLE tax_bills_nyc 
(
    bbl int8,
    owner_name text,
    address text,
    tax_class text,
    tax_rate text,
    emv Float,
    tbea Float,
    bav Float,
    tba text,
    property_tax text,
    condonumber text,
    condo text,
    insertion_date Time 
) SERVER clickhouse_svr;

postgres=# SELECT bbl,tbea,bav,insertion_date FROM tax_bills_nyc LIMIT 5;
    bbl     | tbea  |  bav   | insertion_date 
------------+-------+--------+----------------
4001940057 | 18755 | 145899 | 15:25:42
1016830130 |  2216 |  17238 | 15:25:42
4012850059 | 69562 | 541125 | 15:25:42
1006130061 | 55883 | 434719 | 15:25:42
3033540009 | 33100 | 257490 | 15:25:42
(5 rows)

CREATE TABLE tax_bills ( bbl bigint, owner_name text) ENGINE = MergeTree PARTITION BY bbl ORDER BY (bbl)

INSERT INTO tax_bills SELECT bbl, tbea from tax_bills_nyc LIMIT 100;

EXPLAIN VERBOSE SELECT bbl,tbea,bav,insertion_date FROM tax_bills_nyc LIMIT 5;
                                     QUERY PLAN                                         
--------------------------------------------------------------------------------------------
Limit  (cost=0.00..0.00 rows=1 width=32)
Output: bbl, tbea, bav, insertion_date
 ->  Foreign Scan on public.tax_bills_nyc  (cost=0.00..0.00 rows=0 width=32)
         Output: bbl, tbea, bav, insertion_date
         Remote SQL: SELECT bbl, tbea, bav, insertion_date FROM test_database.tax_bills_nyc
(5 rows)

Aggregate Pushdown.

Aggregate pushdown is a new feature of PostgreSQL FDW. There are currently very few foreign data wrappers that support aggregate pushdown: clickhousedb_fdw is one of them. Planner decides which aggregate to pushdown or not. Here is an example for both cases.

postgres=# EXPLAIN VERBOSE SELECT count(bbl) FROM tax_bills_nyc LIMIT 5;
                                QUERY PLAN                                                                
---------------------------------------------------------------------------
Limit  (cost=0.00..0.01 rows=1 width=8)
Output: (count(bbl))
->  Aggregate  (cost=0.00..0.01 rows=1 width=8)
         Output: count(bbl)
         ->  Foreign Scan on public.tax_bills_nyc  (cost=0.00..0.00 rows=0 width=8)
            Output: bbl, owner_name, address, tax_class, tax_rate, emv, tbea, bav, tba, property_tax, condonumber, condo, insertion_date
               Remote SQL: SELECT bbl FROM test_database.tax_bills_nyc
(7 rows)


    EXPLAIN VERBOSE SELECT count(bbl) FROM tax_bills_nyc;
                            QUERY PLAN                            
------------------------------------------------------------------
Foreign Scan  (cost=1.00..-1.00 rows=1000 width=8)
Output: (count(bbl))
Relations: Aggregate on (tax_bills_nyc)
Remote SQL: SELECT count(bbl) FROM test_database.tax_bills_nyc
(4 rows)
Join Pushdown

Join pushdown is also a very new feature of PostgreSQL FDW’s. The clickhousedb_fdw also supports join pushdown.

EXPLAIN VERBOSE SELECT t2.bbl, t2.owner_name, t1.bav FROM tax_bills_nyc t1 RIGHT OUTER JOIN tax_bills t2 ON (t1.bbl = t2.bbl);
                                                                    QUERY PLAN                                                                     

-------------------------------------------------------------------------------------------------------------------------------------------------------
----
Foreign Scan  (cost=1.00..-1.00 rows=1000 width=50)
Output: t2.bbl, t2.owner_name, t1.bav
Relations: (tax_bills t2) LEFT JOIN (tax_bills_nyc t1)
Remote SQL: SELECT r2.bbl, r2.owner_name, r1.bav FROM  test_database.tax_bills r2 ALL LEFT JOIN test_database.tax_bills_nyc r1 ON (((r1.bbl = r2.bbl
)))
(4 rows)
Limitations

There are some limitations and ToDos. In some instances, more research is needed to implement cases such as these:

1 - All configuration parameters of ClickHouse database.
2 - Complex join pushdown support.
3 - Delete from a ClickHouse partition
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