ivan-zaitsev / sql-example

PostgreSQL setup

1. Setup Postgres server

docker-compose up -d

2. Connect to Postgres server

psql -h 127.0.0.1 -p 5432 -U postgres -d postgres

Practice

Pagination

• OFFSET
• KEYSET (Cursor or Seek)

OFFSET:

EXPLAIN ANALYZE
SELECT * FROM users
ORDER BY id ASC
OFFSET 99990 FETCH NEXT 10 ROWS ONLY;

KEYSET:

EXPLAIN ANALYZE
SELECT * FROM users
WHERE id > 99990
ORDER BY id ASC
LIMIT 10;

ID types

• SEQUENTIAL (Incremental)
• UUID (Random or Sorted)

Transactions

More: https://www.postgresql.org/docs/current/transaction-iso.html

A - Atomicity C - Consistency I - Isolation D - Durability

Isolation levels:

• Read uncommitted
• Read committed
• Repeatable read
• Serializable

Anomalies:

• dirty read
• non-repeatable read
• phantom read
• lost update
• read skew
• write skew

	dirty read	non-repeatable read	phantom read	lost update	read skew	write skew
Read uncommitted	-	+	+	+	+	+
Read committed	-	+	+	+	+	+
Repeatable read	-	-	-	-	-	+
Serializable	-	-	-	-	-	-

Dirty read:

BEGIN TRANSACTION;                                    -- Transaction A
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;     -- Transaction A

UPDATE products SET price = 100 WHERE id = 1;         -- Transaction A

BEGIN TRANSACTION;                                    -- Transaction B

SELECT price FROM products WHERE id = 1;              -- Transaction B: Reads data that transaction A did not commit.
... perofrm some calculations                         -- Transaction B

COMMIT TRANSACTION;                                   -- Transaction A
ROLLBACK TRANSACTION;                                 -- Transaction B: Rollbacks changes, transaction A relied on rollbacked value.

Non-repeatable read:

BEGIN TRANSACTION;                                    -- Transaction A
--SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;    -- Transaction A

SELECT price FROM products WHERE id = 1;              -- Transaction A

BEGIN TRANSACTION;                                    -- Transaction B

UPDATE products SET price = 100 WHERE id = 1;         -- Transaction B

COMMIT TRANSACTION;                                   -- Transaction B

SELECT price FROM products WHERE id = 1;              -- Transaction A: Data was changed by Transaction B, second select has different value.
... perofrm some calculations                         -- Transaction A

COMMIT TRANSACTION;                                   -- Transaction A

Phantom read:

BEGIN TRANSACTION;                                    -- Transaction A
--SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;    -- Transaction A

SELECT * FROM orders WHERE user_id = 50;              -- Transaction A

BEGIN TRANSACTION;                                    -- Transaction B

INSERT INTO orders (user_id, created_at)              -- Transaction B
VALUES (50, CURRENT_TIMESTAMP);                       -- Transaction B

COMMIT TRANSACTION;                                   -- Transaction B

SELECT * FROM orders WHERE user_id = 50;              -- Transaction A: A new record was added by Transaction B, data set is different.
... perofrm some calculations                         -- Transaction A

COMMIT TRANSACTION;                                   -- Transaction A

Lost update:

BEGIN TRANSACTION;                                    -- Transaction A
--SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;    -- Transaction A

SELECT * FROM products WHERE id = 1;                  -- Transaction A

UPDATE products SET price = 100 WHERE id = 1;         -- Transaction A

BEGIN TRANSACTION;                                    -- Transaction B

SELECT * FROM products WHERE id = 1;                  -- Transaction B

UPDATE products SET                                   -- Transaction B
  price = 50, name = '...' WHERE id = 1;              -- Transaction B: Overwrites changes of transaction A

COMMIT TRANSACTION;                                   -- Transaction A
COMMIT TRANSACTION;                                   -- Transaction B

Read skew:

BEGIN TRANSACTION;                                    -- Transaction A
--SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;    -- Transaction A

SELECT * FROM orders WHERE id = 1;                    -- Transaction A

BEGIN TRANSACTION;                                    -- Transaction B

INSERT INTO orders_items(                             -- Transaction B
  order_id, product_id, quantity) VALUES(1, 1, 1);    -- Transaction B

COMMIT TRANSACTION;                                   -- Transaction B

SELECT * FROM orders_items WHERE order_id = 1;        -- Transaction A: A new record was added by Transaction B, data set is different.
... perofrm some calculations                         -- Transaction A

COMMIT TRANSACTION;                                   -- Transaction A

Write skew:

BEGIN TRANSACTION;                                    -- Transaction A
--SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;       -- Transaction A

SELECT SUM(oi.quantity * p.price)                     -- Transaction A
FROM orders_items oi                                  -- Transaction A
JOIN products p ON oi.product_id = p.id               -- Transaction A
WHERE oi.order_id = 1;                                -- Transaction A

BEGIN TRANSACTION;                                    -- Transaction B
--SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;       -- Transaction A

SELECT SUM(oi.quantity * p.price)                     -- Transaction B
FROM orders_items oi                                  -- Transaction B
JOIN products p ON oi.product_id = p.id               -- Transaction B
WHERE oi.order_id = 1;                                -- Transaction B

INSERT INTO orders_items(                             -- Transaction B
  order_id, product_id, quantity) VALUES(1, 1, 1);    -- Transaction B

COMMIT TRANSACTION;                                   -- Transaction B

INSERT INTO orders_items(                             -- Transaction B
  order_id, product_id, quantity) VALUES(1, 1, 1);    -- Transaction B: Transaction A already inserted order item.

COMMIT TRANSACTION;                                   -- Transaction A

Locks

More: https://www.postgresql.org/docs/current/explicit-locking.html

• Optimistic
• Pessimistic

Optimistic:

• Timestamp
• Integer (Version)

Integer:

BEGIN TRANSACTION;                                    -- Transaction A

SELECT * FROM products WHERE id = 1;                  -- Transaction A

UPDATE products SET                                   -- Transaction A
  price = 100, version = version + 1                  -- Transaction A
WHERE id = 1 and version = 1;                         -- Transaction A

BEGIN TRANSACTION;                                    -- Transaction B

SELECT * FROM products WHERE id = 1;                  -- Transaction B

UPDATE products SET                                   -- Transaction B
  price = 50, name = '...', version = version + 1     -- Transaction B
WHERE id = 1 AND version = 1;                         -- Transaction B: Cannot update because version is different

COMMIT TRANSACTION;                                   -- Transaction A
COMMIT TRANSACTION;                                   -- Transaction B

Pessimistic:

• Row level
• Table level

Row level: Read lock (FOR SHARE): allows read, prevents write Write lock (FOR UPDATE): prevents read and write

Example:

BEGIN TRANSACTION;                                    -- Transaction A

SELECT * FROM products WHERE id = 1 FOR SHARE;        -- Transaction A
UPDATE products SET price = 100 WHERE id = 1;         -- Transaction A

BEGIN TRANSACTION;                                    -- Transaction B

SELECT * FROM products WHERE id = 1;                  -- Transaction B

UPDATE products SET                                   -- Transaction B
  price = 50, name = '...' WHERE id = 1;              -- Transaction B: Cannot update because of share lock.

COMMIT TRANSACTION;                                   -- Transaction A
COMMIT TRANSACTION;                                   -- Transaction B