Grokking Algorithm

Identity column

Ushbu qo'llanmada siz jadval uchun PostgreSQL identifikator ustunini yaratish uchun GENERATED AS IDENTITY cheklovidan qanday foydalanishni o'rganasiz.

PostgreSQL 10-versiyasida identifikator sifatida yaratilgan yangi cheklov joriy etildi, bu sizga avtomatik ravishda ustunga noyob raqam belgilash imkonini beradi.

GENERATED AS IDENTITY cheklovi eski SERIAL ustunining SQL standartiga mos variantidir.

Quyida GENERATED AS IDENTITY cheklovining sintaksisi tasvirlangan:

column_name type GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY[ ( sequence_option ) ]

Ushbu sintaksisda:

  • Turi SMALLINT, INT yoki BIGINT bo'lishi mumkin.

  • GENERATED ALWAYS PostgreSQL-ga har doim identifikatsiya ustuni uchun qiymat yaratishni buyuradi. Agar siz qiymatlarni GENERATED ALWAYS AS IDENTITY ustuniga kiritishga (yoki yangilashga) harakat qilsangiz, PostgreSQL xatolik chiqaradi.

  • GENERATED BY BY DEFAULT shuningdek, PostgreSQL-ga identifikatsiya ustuni uchun qiymat yaratishni buyuradi. Biroq, agar siz kiritish yoki yangilash uchun qiymat bersangiz, PostgreSQL tizim tomonidan yaratilgan qiymatdan foydalanish o'rniga identifikatsiya ustuniga kiritish uchun ushbu qiymatdan foydalanadi.

PostgreSQL sizga jadvalda bir nechta identifikatsiya ustuniga ega bo'lish imkonini beradi. SERIAL singari, GENERATED AS IDENTITY cheklovi ham SEQUENCE ob'ektini ichki sifatida ishlatadi.

PostgreSQL identifikatsiya ustuniga misollar

A. GENERATED ALWAYS

Birinchidan, identifikatsiya ustuni sifatida color_id bilan rang nomli jadval yarating:

CREATE TABLE color (
    color_id INT GENERATED ALWAYS AS IDENTITY,
    color_name VARCHAR NOT NULL
);

Ikkinchidan, colot jadvaliga yangi qator qo'shing:

INSERT INTO color(color_name)
VALUES ('Red');

color_id ustunida GENERATED AS IDENTITY cheklovi mavjud bo'lganligi sababli, PostgreSQL quyidagi so'rovda ko'rsatilganidek, uning qiymatini yaratadi:

SELECT * FROM color;
output

Uchinchidan, color_id va color_name ustunlari uchun qiymatlarni kiritish orqali yangi qator qo'shing:

INSERT INTO color (color_id, color_name)
VALUES (2, 'Green');

PostgreSQL quyidagi xatoni chiqardi:

[Err] ERROR:  cannot insert into column "color_id"
DETAIL:  Column "color_id" is an identity column defined as GENERATED ALWAYS.
HINT:  Use OVERRIDING SYSTEM VALUE to override.

Xatoni tuzatish uchun siz OVERRIDING SYSTEM VALUE bandidan quyidagi tarzda foydalanishingiz mumkin:

INSERT INTO color (color_id, color_name)
OVERRIDING SYSTEM VALUE 
VALUES(2, 'Green');
output

Yoki uning o'rniga GENERATED BY BY DEFAULT AS IDENTITY dan foydalaning.

B. GENERATED BY DEFAULT AS IDENTITY

Birinchidan, color jadvalini o'chiring va uni qayta yarating. Bu safar biz GENERATED BY DEFAULT AS IDENTITY dan foydalanamiz:

DROP TABLE color;

CREATE TABLE color (
    color_id INT GENERATED BY DEFAULT AS IDENTITY,
    color_name VARCHAR NOT NULL
);

Ikkinchidan, color jadvaliga qator qo'shing:

INSERT INTO color (color_name)
VALUES ('White');

Kutilganidek ishlaydi.

Uchinchidan, color_id ustuni qiymatiga ega boshqa qatorni kiriting:

INSERT INTO color (color_id, color_name)
VALUES (2, 'Yellow');

GENERATED ALWAYS AS IDENTITY cheklovidan foydalanadigan oldingi misoldan farqli o'laroq, yuqoridagi bayonot juda yaxshi ishlaydi.

C. Ketma-ket variantlarga misol

GENERATED AS IDENTITY cheklovi SEQUENCE obyektidan foydalanganligi sababli tizim tomonidan yaratilgan qiymatlar uchun ketma-ketlik parametrlarini belgilashingiz mumkin.

Masalan, siz boshlang'ich qiymat va o'sishni quyidagicha belgilashingiz mumkin:

DROP TABLE color;

CREATE TABLE color (
    color_id INT GENERATED BY DEFAULT AS IDENTITY 
    (START WITH 10 INCREMENT BY 10),
    color_name VARCHAR NOT NULL
);

Ushbu misolda color_id ustuni uchun tizim tomonidan yaratilgan qiymat 10 dan boshlanadi va o'sish qiymati ham 10 ga teng.

Birinchidan, color jadvaliga yangi qator qo'shing:

INSERT INTO color (color_name)
VALUES ('Orange');

color_id ustunining boshlang'ich qiymati quyida ko'rsatilgandek o'nga teng:

SELECT * FROM color;
output

Ikkinchidan, color jadvaliga boshqa qatorni kiriting:

INSERT INTO color (color_name)
VALUES ('Purple');

O'sish opsiyasi tufayli ikkinchi qatorning color_id qiymati 20 ga teng.

SELECT * FROM color;
output

Mavjud jadvalga identifikatsiya ustunini qo'shish

ALTER TABLE iborasining quyidagi shaklidan foydalanib, mavjud jadvalga identifikatsiya ustunlarini qo'shishingiz mumkin:

ALTER TABLE table_name 
ALTER COLUMN column_name 
ADD GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY { ( sequence_option ) }

Keling, quyidagi misolni ko'rib chiqaylik. Birinchidan, shape nomli yangi jadval yarating:

CREATE TABLE shape (
    shape_id INT NOT NULL,
    shape_name VARCHAR NOT NULL
);

Ikkinchidan, shape_id ustunini identifikatsiya ustuniga o'zgartiring:

ALTER TABLE shape 
ALTER COLUMN shape_id ADD GENERATED ALWAYS AS IDENTITY;

E'tibor bering, shape_id identifikatsiya ustuniga o'zgartirilishi uchun NOT NULL chekloviga ega bo'lishi kerak. Aks holda, siz quyidagi xatolikni olasiz:

ERROR:  column "shape_id" of relation "shape" must be declared NOT NULL before identity can be added
SQL state: 55000

Quyidagi buyruq psql asbobidagi shape jadvalini tavsiflaydi:

\d shape

U biz kutgan quyidagi natijani qaytaradi:

Identifikatsiya ustunini o'zgartirish

Mavjud identifikatsiya ustunining xususiyatlarini quyidagi ALTER TABLE iborasidan foydalanib o'zgartirishingiz mumkin:

ALTER TABLE table_name 
ALTER COLUMN column_name 
{ SET GENERATED { ALWAYS| BY DEFAULT } | 
  SET sequence_option | RESTART [ [ WITH ] restart ] }

Misol uchun, quyidagi bayonot shape jadvalining shape_id ustunini GENERATED BY BY DEFAULT ga o'zgartiradi:

ALTER TABLE shape
ALTER COLUMN shape_id SET GENERATED BY DEFAULT;

Quyidagi buyruq psql asbobidagi shakllar jadvalining tuzilishini tavsiflaydi:

\d shape
output

Natijadan ko'rinib turibdiki, shape_id ustuni GENERATED ALWAYS dan GENERATED BY BY DEFAULT ga o'zgartirildi.

GENERATED AS IDENTITY cheklovini olib tashlash

Quyidagi bayonot mavjud jadvaldan GENERATED AS IDENTITY cheklovini olib tashlaydi:

ALTER TABLE table_name 
ALTER COLUMN column_name 
DROP IDENTITY [ IF EXISTS ]

Masalan, siz shape jadvalining shape_id ustunidan GENERATED AS IDENTITY cheklash ustunini quyidagicha olib tashlashingiz mumkin:

ALTER TABLE shape
ALTER COLUMN shape_id
DROP IDENTITY IF EXISTS;

Ushbu qo'llanmada siz PostgreSQL identifikator ustunidan qanday foydalanishni va uni GENERATED AS IDENTITY cheklovi yordamida qanday boshqarishni o'rgandingiz.

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