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Explain Explaining the query will help you to estimate how expensive your query is and which query plan will be used. 1 2 3 4 5 6 khanh=# explain select g from grades where g = 100; QUERY PLAN ------------------------------------------------------------------------------- Index Only Scan using g_index on grades (cost=0.42..97.55 rows=4636 width=4) Index Cond: (g = 100) (2 rows) So, this query will use Index-Only-Scan. The cost of the query is ranging from 0.42 to 97.55, 4636 is number of estimated return rows, width is size of return data in byte. ...

Check timezone of the current session 1 2 3 4 5 khanh=# show timezone; TimeZone ---------- UTC (1 row) Change timezone of a session 1 2 3 4 5 6 7 khanh=# set timezone='asia/ho_chi_minh'; SET khanh=# show timezone; TimeZone ------------------ Asia/Ho_Chi_Minh (1 row) you can get timezone names here: https://en.wikipedia.org/wiki/List_of_tz_database_time_zones timestamp and timestamptz timestamptz aka timestamp with time zone, the time when return to the client will be converted to the timezone has picked in the session. ...

What is SSL? and why do we need them? SSL is a method to encrypt your data before transmitting it to the receiver. If we don’t encrypt and send data as plain text, somebody can sniff in your connection to read all your messages. How do SSL works? Of course, to establish an SSL, we need a TCP connection first, let’s assume that we have a TCP connection ready. How an SSL connection is established ...

No! Not really! NodeJS has one main thread called event-loop and it also has a worker pool to handle blocking IO and CPU-intensive tasks. The default number of threads in the worker pool is 4, and you can change it. Node.js uses a small number of threads to handle many clients. In Node.js there are two types of threads: one Event Loop (aka the main loop, main thread, event thread, etc.), and a pool of k Workers in a Worker Pool (aka the threadpool). ...

To demonstrate what is the difference between object hash code and object equal, I would like to start with a simple example in Python. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 class Person: def __init__(self, name: str, age: int) -> None: self.name = name self.age = age def __repr__(self) -> str: return f'{self.name} {self.age}' john = Person('John', 18) eventRegister = {} # John want to join the event eventRegister[john] = True # In the next day, # he changes his mind john = Person('John', 18) eventRegister[john] = False print(f'registers: {eventRegister}') # Check john = Person('John', 18) print(f'Will John join the event? {eventRegister[john]}') It’s quite simple, right? I define a new Person class that has two properties age and name. And then I defined a map, to check if a person join the event or not. ...

B tree and B+ tree B tree is a self-balance tree. I can see that the B tree and AVL tree have a thing in common – it is all self-balance. But the difference is, each node in the AVL tree store exactly one value, and have at most two children. Each node of the B tree contains an array of at most N values and has at most N + 1 children. ...

1. Read phenomena When transaction A reads the data that might be changed by transaction B. 1.1 Dirty reads: Is when a transaction read uncommitted data from another transaction. Example: Transaction A Transaction B begin transaction; select age from employee where id = 1; // age = 24 begin transaction; update employee set age = 25 where id = 1; // age = 25 select age from employee where id = 1; // age = 25 rollback; select age from employee where id = 1; // age = 24 commit; 1.2 Non-repeatable reads When during a transaction, you retrieve a row two times, and the second time, you got a slightly different row. It is different from the dirty read in that this time, it read committed data. ...

Atomicity: Each transaction could have multiple steps in it, each step might be a query or an update to the data. Atomicity means that the transaction is considered a success if all steps are a success. If just one of those steps is failed, the whole transaction is considered to fail, and it has to roll back to the previous state. Consistency: In the database, it might have some rules or constraints, and the transaction when modified data have to follow those constraints. For example, let say the constrain for bank account balance is 0 or positive, so if a transaction tries to assign a negative number to a bank account, it violates the constrain and has to roll back. ...

So this is just a quick recap of what I have learned about networking First, you have to connect your computer to the local network. Your computer then will be assigned a local IP by a DHCP server (Dynamic Host Configuration Protocol), you cannot use that local IP to talk to the outside network. All the devices in the same local network will use the IP of the gateway to talk to the internet, sometimes it’s called public IP. The protocol to map from local address to public address is called NAT (Network Address Translation). ...