--- /dev/null
+Extension is a set of PostgreSQL objects that can be `CREATE`ed.
+Like `CREATE FUNCTION`, `CREATE TYPE` or `CREATE OPERATOR`.
+This set is created, updated and deleted as a bundle.
+When you do `CREATE EXTENSION` you it will create all object included into extension.
+When you do `DROP EXTENSION` all that objects will be automatically removed.
+
+So extension is just a set of SQL commands to create or update extension schema.
+(You even do not need to delete it, `DROP EXTENSION` will delete them automatically)
+
+Functions may be written in `PL/pgSQL` or other `PL/*` or you can load binary function from shared `.so` library.
+.
+You can register a function as a hook handler to change behavior if PostgreSQL.
+
+
+
+See https://github.com/IshaanAdarsh/Postgres-extension-tutorial/blob/main/SGML/Main.md for more info
+
+```
+
+$ whoami
+nataraj
+
+$ sudo -u postgres createuser -s nataraj
+```
+
+Make sure that `-dev` files for postgres are installed. In Debian-based distributives it wil
+be like this
+
+```
+sudo apt-get install postgresql-server-dev-all
+
+```
+
+
+```
+$ mkdir my_first_extension
+
+$ cd my_first_extension
+```
+
+```
+touch my_first_extension.control
+```
+
+```
+# my_first_extension.control
+# Comment line to provide additional information about the extension
+comment = 'My Very First Extension'
+
+# Specifies the default version of the extension
+default_version = '0.0.1'
+```
+
+https://www.postgresql.org/docs/17/extend-extensions.html#EXTEND-EXTENSIONS-FILES
+
+
+---
+
+```
+touch my_first_extension--0.0.1.sql
+```
+
+```
+CREATE TABLE my_first_extension_table (
+ id SERIAL PRIMARY KEY,
+ my_string VARCHAR NOT NULL
+);
+
+INSERT INTO my_first_extension_table (my_string) VALUES('Hello, World');
+```
+
+---
+
+```
+touch Makefile
+```
+
+```
+EXTENSION = my_first_extension # Name of the extension
+DATA = my_first_extension--0.0.1.sql # SQL file containing extension objects and functions
+
+PG_CONFIG ?= pg_config # Path to the pg_config executable
+PGXS := $(shell $(PG_CONFIG) --pgxs) # Get the PostgreSQL Extension Makefile from pg_config
+include $(PGXS) # Include the PostgreSQL Extension Makefile
+```
+
+---
+
+```
+make
+sudo make install
+```
+
+```
+sudo -u postgres psql
+
+# CREATE EXTENSION my_first_extension;
+CREATE EXTENSION
+
+# SELECT * FROM my_first_extension_table;
+ id | my_string
+----+--------------
+ 1 | Hello, World
+(1 row)
+
+# DROP EXTENSION my_first_extension;
+DROP EXTENSION
+
+# SELECT * FROM my_first_extension_table;
+ERROR: relation "my_first_extension_table" does not exist
+СТРОКА 1: SELECT * FROM my_first_extension_table;
+ ^
+```
+
+------------
+
+# Add tests
+
+```
+mkdir sql
+```
+
+edit `sql/my_first_extension-main-test.sql`
+
+```
+CREATE EXTENSION my_first_extension;
+
+SELECT * FROM my_first_extension_table;
+
+DROP EXTENSION
+
+-- There should be no my_first_extension_table anymore
+SELECT * FROM my_first_extension_table;
+```
+
+----
+
+change Makefile
+
+```
+EXTENSION = my_first_extension # Name of the extension
+DATA = my_first_extension--0.0.1.sql # SQL file containing extension objects and functions
+REGRESS = my_first_extension-main-test
+
+
+PG_CONFIG ?= pg_config # Path to the pg_config executable
+PGXS := $(shell $(PG_CONFIG) --pgxs) # Get the PostgreSQL Extension Makefile from pg_config
+include $(PGXS) # Include the PostgreSQL Extension Makefile
+```
+
+run `make installcheck`
+
+
+and get an error: `my_first_extension/expected/my_first_extension-main-test.out: No such file or directory`
+
+and a `results` dir.
+
+----
+
+Check that `results/my_first_extension-main-test.out` is what you expect
+
+```
+$ cat results/my_first_extension-main-test.out
+CREATE EXTENSION my_first_extension;
+SELECT * FROM my_first_extension_table;
+ id | my_string
+----+--------------
+ 1 | Hello, World
+(1 row)
+
+DROP EXTENSION
+-- There should be no my_first_extension_table anymore
+SELECT * FROM my_first_extension_table;
+ERROR: syntax error at or near "SELECT"
+LINE 3: SELECT * FROM my_first_extension_table;
+ ^
+```
+
+----
+
+If everything as you expected copy `my_first_extension-main-test.out` from `results` dir to `expected`
+
+```
+mkdir expected
+cp results/my_first_extension-main-test.out expected
+```
+
+----
+
+Now tests should pass
+
+```
+n$ make installcheck
+echo "+++ regress install-check in +++" && /usr/lib/postgresql/15/lib/pgxs/src/makefiles/../../src/test/regress/pg_regress --inputdir=./ --bindir='/usr/lib/postgresql/15/bin' --dbname=contrib_regression my_first_extension-main-test
++++ regress install-check in +++
+(using postmaster on Unix socket, default port)
+============== dropping database "contrib_regression" ==============
+SET
+DROP DATABASE
+============== creating database "contrib_regression" ==============
+CREATE DATABASE
+ALTER DATABASE
+ALTER DATABASE
+ALTER DATABASE
+ALTER DATABASE
+ALTER DATABASE
+ALTER DATABASE
+============== running regression test queries ==============
+test my_first_extension-main-test ... ok 54 ms
+
+=====================
+ All 1 tests passed.
+=====================
+```
+
+-----
+
+Now let's create create a new version of extension, with a plpsql function in it
+
+* Change version from `0.0.1` to `0.0.2` in `my_first_extension.control`:
+
+```
+default_version = '0.0.2'
+```
+
+* rename schema creation file from `my_first_extension--0.0.1.sql` to `my_first_extension--0.0.2.sql`
+* create an schema update file `my_first_extension--0.0.1--0.0.2.sql`
+
+Add to both of them function creation code
+
+```
+CREATE FUNCTION my_one()
+returns int
+language plpgsql
+as
+$$
+begin
+ return 1;
+end;
+$$;
+
+```
+
+* Update `Makefile` adding new sql file names to `DATA` variable
+
+```
+DATA = my_first_extension--0.0.2.sql my_first_extension--0.0.1--0.0.2.sql
+```
+
+* install extension
+
+```
+sudo make install
+
+```
+
+* check everything works
+
+```
+$ sudo -u postgres psql
+
+=# CREATE EXTENSION my_first_extension;
+CREATE EXTENSION
+
+s=# SELECT my_one();
+ my_one
+--------
+ 1
+(1 row)
+```
+
+----
+
+Write test for `my_one` function yourself
+
+----
+
+https://www.highgo.ca/2020/01/10/how-to-create-test-and-debug-an-extension-written-in-c-for-postgresql/
+
+
+Creating C-function
+
+// Do not forget to install `postgresql-server-dev-all`
+
+create `my_first_extension.c` file
+
+```
+#include "postgres.h"
+#include "fmgr.h"
+
+PG_MODULE_MAGIC;
+
+PG_FUNCTION_INFO_V1(my_get_two);
+
+Datum
+my_get_two(PG_FUNCTION_ARGS)
+{
+ PG_RETURN_INT32(2);
+}
+```
+
+add following lines to `Makefile`
+
+```
+MODULE_big = my_first_extension
+OBJS = my_first_extension.o
+
+```
+and to the `my_first_extension.control`
+
+
+```
+module_pathname = '$libdir/my_first_extension'
+
+```
+
+
+Check everything works well by running
+
+```
+make
+```
+
+add to schema
+```
+CREATE FUNCTION my_two()
+RETURNS INT
+AS 'MODULE_PATHNAME', 'my_get_two'
+LANGUAGE C;
+```
+
+
+bump version in *.sql files, Makefile and control file
+
+
+```
+make
+sudo make install
+```
+
+Check everything works:
+
+```
+$ sudo -u postgres psql
+
+=# DROP EXTENSION my_first_extension;
+DROP EXTENSION
+
+=# CREATE EXTENSION my_first_extension;
+CREATE EXTENSION
+postgres=# SELECT my_two();
+ my_two
+--------
+ 2
+(1 row)
+
+```
+
+Do not forget to write tests!
+
+
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