SQLite Foreign Data Wrapper for PostgreSQL

This is a foreign data wrapper (FDW) to connect PostgreSQL to SQLite database file. This FDW works with PostgreSQL 11, 12, 13, 14, 15 and confirmed with SQLite 3.38.5.

Features
Supported platforms
Installation
Usage
Functions
Identifier case handling
Generated columns
Character set handling
Examples
Limitations
Tests
Contributing
Useful links

Features

Common features

Transactions
Support INSERT/UPDATE/DELETE (both Direct modification and Foreign modification).
Support TRUNCATE by deparsing into DELETE statement without WHERE clause
Allow control over whether foreign servers keep connections open after transaction completion. This is controlled by keep_connections and defaults to on
Support list cached connections to foreign servers by using function sqlite_fdw_get_connections()
Support discard cached connections to foreign servers by using function sqlite_fdw_disconnect(), sqlite_fdw_disconnect_all().
Support Bulk INSERT by using batch_size option
Support INSERT/UPDATE with generated column
Support ON CONFLICT DO NOTHING.

Pushdowning

WHERE clauses are pushdowned
Aggregate function are pushdowned
ORDER BY is pushdowned
Joins (left/right/inner/cross) are pushdowned
CASE expressions are pushdowned.
LIMIT and OFFSET are pushdowned (*when all tables queried are fdw)
Support GROUP BY, HAVING push-down.
upper, lower and other character case functions are not pushed down because they does not work with UNICODE character in SQLite.
WITH TIES option is not pushed down.

Notes about pushdowning

For push-down case, the number after floating point may be different from the result of PostgreSQL.

Notes about features

SQLite evaluates division by zero as NULL. It is different from PostgreSQL, which will display Division by zero error.
The data type of column of foreign table should match with data type of column in SQLite to avoid wrong result. For example, if the column of SQLite is float (which will be stored as float8), the column of foreign table should be float8, too. If the column of foreign table is float4, it may cause wrong result when SELECT.
For key option, user needs to specify the primary key column of SQLite table corresponding with the key option. If not, wrong result may occur when UPDATE or DELETE.
When Sum of data in table is out of range, sqlite_fdw will display Infinity value. It is different from PostgreSQL FDW, which will display ERROR: value out of range: overflow error.
For numeric data type, sqlite_fdw use sqlite3_column_double to get value, while SQLite shell uses sqlite3_column_text to get value. Those 2 APIs may return different numeric value. Therefore, for numeric data type, the value returned from sqlite_fdw may different from the value returned from SQLite shell.
sqlite_fdw can return implementation-dependent order for column if the column is not specified in ORDER BY clause.
When the column type is varchar array, if the string is shorter than the declared length, values of type character will be space-padded; values of type character varying will simply store the shorter string.

Also see Limitations

Supported platforms

sqlite_fdw was developed on Linux and should run on any reasonably POSIX-compliant system.

Installation

Package installation

For some Linux distributives internal packages with sqlite_fdw are avalilable.

sqlite_fdw_14 rpm for CentOS 9, RHEL 9, Rocky Linux 9, AlmaLinux 9. Also there is other versions.
sqlite_fdw git package for Arch Linux.

Source installation

Prerequisites:

libsqlite3-dev, especially sqlite.h
postgresql-server-dev, especially postgres.h
gcc
make

1. Install SQLite & Postgres Development Libraries

For Debian or Ubuntu: apt-get install libsqlite3-dev apt-get install postgresql-server-dev-XX, where XX matches your postgres version, i.e. apt-get install postgresql-server-dev-15

You can also download SQLite source code and build SQLite.

2. Build and install sqlite_fdw

Add a directory of pg_config to PATH and build and install sqlite_fdw.

make USE_PGXS=1
make install USE_PGXS=1

If you want to build sqlite_fdw in a source tree of PostgreSQL, use

make
make install

Usage

CREATE SERVER options

sqlite_fdw accepts the following options via the CREATE SERVER command:

database as string, required

SQLite database path.
updatable as boolean, optional, default true

This option allow or disallow write operations on SQLite database file.
truncatable as boolean, optional, default true

Allows foreign tables to be truncated using the TRUNCATE command.
keep_connections as boolean, optional, default true

Allows to keep connections to SQLite while there is no SQL operations between PostgreSQL and SQLite.
batch_size as integer, optional, default 1

Specifies the number of rows which should be inserted in a single INSERT operation. This setting can be overridden for individual tables.

CREATE USER MAPPING options

There is no user or password conceptions in SQLite, hence sqlite_fdw no need any CREATE USER MAPPING command.

In OS sqlite_fdw works as executed code with permissions of user of PostgreSQL server. Usually it is postgres OS user. For interacting with SQLite database without access errors ensure this user have follow permissions:

read permission on all directories by path to the SQLite database file;
read permission on SQLite database file;
write permissions both on SQLite database file and directory it contains if you need a modification. During INSERT, UPDATE or DELETE in SQLite database, SQLite engine functions makes temporary files with transaction data in the directory near SQLite database file. Hence without write permissions you'll have a message failed to execute remote SQL: rc=8 attempt to write a readonly database.

CREATE FOREIGN TABLE options

sqlite_fdw accepts the following table-level options via the CREATE FOREIGN TABLE command:

table as string, optional, no default

SQLite table name. Use if not equal to name of foreign table in PostgreSQL. Also see about identifier case handling.
truncatable as boolean, optional, default from the same CREATE SERVER option

See CREATE SERVER options section for details.
batch_size as integer, optional, default from the same CREATE SERVER option

See CREATE SERVER options section for details.
updatable as boolean, optional, default true

This option can allow or disallow write operations on a SQLite table independed of the same server option.

sqlite_fdw accepts the following column-level options via the CREATE FOREIGN TABLE command:

column_name as string, optional, no default

This option gives the column name to use for the column on the remote server. Also see about identifier case handling.
column_type as string, optional, no default

Option to convert INT SQLite column (epoch Unix Time) to be treated/visualized as TIMESTAMP in PostgreSQL.
key as boolean, optional, default false

Indicates a column as a part of primary key or unique key of SQLite table.

IMPORT FOREIGN SCHEMA options

sqlite_fdw supports IMPORT FOREIGN SCHEMA (PostgreSQL 9.5+) and accepts no custom options for this command.

TRUNCATE support

sqlite_fdw implements the foreign data wrapper TRUNCATE API, available from PostgreSQL 14.

As SQlite does not provide a TRUNCATE command, it is simulated with a simple unqualified DELETE operation.

Actually, TRUNCATE ... CASCADE can be simulated if we create child table of SQLite with foreign keys and ON DELETE CASCADE, and then executing TRUNCATE (which will be deparsed to DELETE).

Following restrictions apply:

TRUNCATE ... RESTART IDENTITY is not supported
SQLite tables with foreign key references can cause errors during truncating

Functions

As well as the standard sqlite_fdw_handler() and sqlite_fdw_validator() functions, sqlite_fdw provides the following user-callable utility functions:

SETOF record sqlite_fdw_get_connections(server_name text, valid bool)
bool sqlite_fdw_disconnect(text)

Closes connection from PostgreSQL to SQLite in the current session.
bool sqlite_fdw_disconnect_all()
sqlite_fdw_version(); Returns standard "version integer" as major version * 10000 + minor version * 100 + bugfix.

sqlite_fdw_version 
--------------------
              20300

Identifier case handling

PostgreSQL folds identifiers to lower case by default, SQlite is case insensetive by default. It's important to be aware of potential issues with table and column names.

This SQL isn't correct for SQLite: Error: duplicate column name: a, but is correct for PostgreSQL

	CREATE TABLE T (
	  "A" INTEGER,
	  "a" NUMERIC
	);

For SQLite there is no difference between

	SELECT * FROM t;   -- №1
	SELECT * FROM T;   -- №2
	SELECT * FROM "t"; -- №3
	SELECT * FROM "T"; -- №4

For PostgreSQL the query with comment №4 is independend query to table T, not to table t as other queries.

If there is

	CREATE TABLE T (
	  A INTEGER,
	  b REAL
	);

in SQLite, both a and A , b and B columns will have the same real datasource in SQlite in follow foreign table:

	CREATE FOREIGN TABLE "SQLite test" (
	  "A" int4 NULL,
	  "B" float8 NULL,
	  "a" int8 NULL,
	  "b" numeric NULL
	)
	SERVER sqlite_server
	OPTIONS (table 'T');

Generated columns

SQLite provides support for generated columns. Behaviour of sqlite_fdw with this columns isn't yet described.

Note that while sqlite_fdw will INSERT or UPDATE the generated column value in SQLite, there is nothing to stop the value being modified within SQLite, and hence no guarantee that in subsequent SELECT operations the column will still contain the expected generated value. This limitation also applies to postgres_fdw.

For more details on generated columns see:

Character set handling

When sqlite_fdw connects to a SQLite no character set metadata stored in SQLite. There is only PRAGMA encoding; with UTF-only values (UTF-8, UTF-16, UTF-16le, UTF-16be). All strings are interpreted acording the PostgreSQL database's server encoding. It's not a problem if both PostgreSQL database and SQLite character data from database file has UTF-8 or UTF-16 encoding. Otherewise character interpretation transformation problems will occur.

Character case functions such as upper, lower and other are not pushed down because they does not work with UNICODE character in SQLite.

Examples

Install the extension:

Once for a database you need, as PostgreSQL superuser.

	CREATE EXTENSION sqlite_fdw;

Create a foreign server with appropriate configuration:

Once for a foreign datasource you need, as PostgreSQL superuser. Please specify SQLite database path using database option.

	CREATE SERVER sqlite_server
	FOREIGN DATA WRAPPER sqlite_fdw
	OPTIONS (
          database '/path/to/database'
	);

Grant usage on foreign server to normal user in PostgreSQL:

Once for a normal user (non-superuser) in PostgreSQL, as PostgreSQL superuser. It is a good idea to use a superuser only where really necessary, so let's allow a normal user to use the foreign server (this is not required for the example to work, but it's secirity recomedation).

	GRANT USAGE ON FOREIGN SERVER sqlite_server TO pguser;

Where pguser is a sample user for works with foreign server (and foreign tables).

User mapping

There is no user or password conceptions in SQLite, hence sqlite_fdw no need any CREATE USER MAPPING command. About access problems see in CREATE USER MAPPING options.

Create foreign table

All CREATE FOREIGN TABLE SQL commands can be executed as a normal PostgreSQL user if there were correct GRANT USAGE ON FOREIGN SERVER. No need PostgreSQL supersuer for secirity reasons but also works with PostgreSQL supersuer.

Please specify table option if SQLite table name is different from foreign table name.

	CREATE FOREIGN TABLE t1 (
	  a integer,
	  b text
	)
	SERVER sqlite_server
	OPTIONS (
	  table 't1_sqlite'
	);

If you want to update tables, please add OPTIONS (key 'true') to a primary key or unique key like the following:

	CREATE FOREIGN TABLE t1(
	  a integer OPTIONS (key 'true'),
	  b text
	)
	SERVER sqlite_server 
	OPTIONS (
	  table 't1_sqlite'
	);

If you need to convert INT SQLite column (epoch Unix Time) to be treated/visualized as TIMESTAMP in PostgreSQL, please add OPTIONS (column_type 'INT') when defining FOREIGN table at PostgreSQL like the following:

	CREATE FOREIGN TABLE t1(
	  a integer,
	  b text,
	  c timestamp without time zone OPTIONS (column_type 'INT')
	)
	SERVER sqlite_server
	OPTIONS (
	  table 't1_sqlite'
	);

As above, but with aliased column names:

	CREATE FOREIGN TABLE t1(
	  a integer,
	  b text OPTIONS (column_name 'test_id'),
	  c timestamp without time zone OPTIONS (column_type 'INT', column_name 'unixtime')
	)
	SERVER sqlite_server
	OPTIONS (
	  table 't1_sqlite'
	);

Import a SQLite database as schema to PostgreSQL:

	IMPORT FOREIGN SCHEMA someschema
	FROM SERVER sqlite_server
	INTO public;

Note: someschema has no particular meaning and can be set to an arbitrary value.

Access foreign table

For the table from previous examples

	SELECT * FROM t1;

Limitations

SQL commands

COPY command for foreign tables is not supported
IMPORT of generated column is not supported
INSERT into a partitioned table which has foreign partitions is not supported. Error Not support partition insert will display.
TRUNCATE in sqlite_fdw always delete data of both parent and child tables (no matter user inputs TRUNCATE table CASCADE or TRUNCATE table RESTRICT) if there are foreign-keys references with ON DELETE CASCADE clause.
RETURNING is not supported.

Arrays

sqlite_fdw only supports ARRAY const, for example, ANY (ARRAY[1, 2, 3]) or ANY ('{1, 2 ,3}').
sqlite_fdw does not support ARRAY expression, for example, ANY (ARRAY[c1, 1, c1+0]).
For ANY(ARRAY) clause, sqlite_fdw deparses it using IN operator.

Numbers (range and precision)

For sum function of SQLite, output of sum(bigint) is integer value. If input values are big, the overflow error may occurs on SQLite because it overflow within the range of signed 64bit. For PostgreSQL, it can calculate as over the precision of bigint, so overflow does not occur.
SQLite promises to preserve the 15 most significant digits of a floating point value. The big value which exceed 15 most significant digits may become different value after inserted.
SQLite does not support numeric type as PostgreSQL. Therefore, it does not allow to store numbers with too high precision and scale. Error out of range occurs.
SQLite does not support special values for IEEE 754-2008 numbers such as NaN, +Infinity and -Infinity in SQL expressions with numeric context. Also SQLite can not store this values with real affinity. In opposite to SQLite, PostgreSQL can store special values in columns belongs to real datatype family such as float or double precision and use arithmetic comparation for this values. In oppose to PostgreSQL, SQLite stores NaN, +Infinity and -Infinity as a text values. Also conditions with special literals (such as n < '+Infinity' or m > '-Infinity' ) isn't numeric conditions in SQLite and gives unexpected result after pushdowning in oppose to internal PostgreSQL calculations. During INSERT INTO ... SELECT or in WHERE conditions sqlite_fdw uses given by PostgreSQL standard case sensetive literals only in follow forms: NaN, -Infinity, Infinity, not original strings from WHERE condition. This can caused selecting issues.

Boolean values

sqlite_fdw boolean values support exists only for bool columns in foreign table. SQLite documentation recommends to store boolean as value with integer affinity. NULL isn't converted, 1 converted to true, all other NOT NULL values converted to false. During SELECT ... WHERE condition_column condition converted only to condition_column.
sqlite_fdw don't provides limited support of boolean values if bool column in foreign table mapped to SQLite text affinity.

Tests

Test directory have structure as following:

+---sql
|   +---11.7
|   |       filename1.sql
|   |       filename2.sql
|   | 
|   +---12.12
|   |       filename1.sql
|   |       filename2.sql
|   | 
.................  
|   \---15.0
|          filename1.sql
|          filename2.sql
|          
\---expected
|   +---11.7
|   |       filename1.out
|   |       filename2.out
|   | 
|   +---12.12
|   |       filename1.out
|   |       filename2.out
|   | 
.................  
|   \---15.0
            filename1.out
            filename2.out

The test cases for each version are based on the test of corresponding version of PostgreSQL. You can execute test by test.sh directly. The version of PostgreSQL is detected automatically by $(VERSION) variable in Makefile. The corresponding sql and expected directory will be used to compare the result. For example, for Postgres 15.0, you can execute "test.sh" directly, and the sql/15.0 and expected/15.0 will be used to compare automatically.

Contributing

Opening issues and pull requests on GitHub are welcome. For pull request, please make sure these items below for testing:

Create test cases (if needed) for the latest version of PostgreSQL supported by sqlite_fdw.
Execute test cases and update expectations for the latest version of PostgreSQL
Test creation and execution for other PostgreSQL versions are welcome but not required.

Useful links

Source

General FDW Documentation

Other FDWs

License

Permission to use, copy, modify, and distribute this software and its documentation for any purpose, without fee, and without a written agreement is hereby granted, provided that the above copyright notice and this paragraph and the following two paragraphs appear in all copies.

See the License file for full details.

pgspider/sqlite_fdw

pgspider

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