PostgreSQL with Deno - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the deno-postgres module in Deno?

The deno-postgres module allows Deno applications to connect to and interact with PostgreSQL databases using SQL queries.

intermediate

How do you create a connection pool to PostgreSQL in Deno?

You create a connection pool by importing Pool from deno-postgres and initializing it with connection options like user, password, database, hostname, and pool size.

intermediate

What is the advantage of using a connection pool with PostgreSQL in Deno?

A connection pool manages multiple database connections efficiently, allowing reuse of connections and improving performance by reducing the overhead of opening and closing connections repeatedly.

beginner

How do you execute a simple SQL query to fetch data from PostgreSQL using Deno?

You acquire a client from the pool, use client.queryObject() with your SQL SELECT statement, then release the client back to the pool after fetching results.

beginner

Why is it important to release the client back to the pool after a query in Deno?

Releasing the client returns it to the pool so other parts of the app can reuse the connection, preventing connection leaks and ensuring efficient resource use.

Which Deno module is commonly used to connect to PostgreSQL?

A deno-sqlite

B deno-postgres

C deno-mysql

D deno-mongodb

The deno-postgres module is designed specifically for PostgreSQL connections in Deno.

What method is used to run a SQL query that returns rows in deno-postgres?

A client.queryObject()

B client.execute()

C client.run()

D client.fetch()

client.queryObject() runs a query and returns results as objects.

Why should you use a connection pool instead of opening a new connection for each query?

A To automatically create tables

B To avoid writing SQL queries

C To reuse connections and improve performance

D To encrypt data

Connection pools reuse existing connections, reducing overhead and improving speed.

What happens if you forget to release a client back to the pool?

A Connections may leak and exhaust the pool

B The database will delete data

C The app will automatically reconnect

D Nothing, it is optional

Not releasing clients causes connection leaks, which can block new queries.

Which of these is NOT a typical connection option for deno-postgres Pool?

A hostname

B user

C database

D fontSize

fontSize is unrelated to database connection settings.

Explain how to set up and use a PostgreSQL connection pool in a Deno application.

Think about the steps from importing to running queries and releasing connections.

Describe why connection pooling is important when working with PostgreSQL in Deno.

Consider resource management and app speed.

Practice

1. What is the main purpose of using the Client from the postgres module in Deno?

easy

A. To manage file uploads in Deno

B. To create a web server in Deno

C. To connect to a PostgreSQL database and run queries

D. To handle user authentication in Deno

Solution

1. Step 1: Understand the role of Client in postgres module

   The Client is designed to establish a connection to a PostgreSQL database and execute SQL queries.

2. Step 2: Compare with other options

   Creating web servers, managing file uploads, and handling authentication are unrelated to the Client in the postgres module.

3. Final Answer:

   To connect to a PostgreSQL database and run queries -> Option C

4. Quick Check:

   Client connects to DB and runs queries = C [OK]

Hint: Remember: Client is for DB connection and queries [OK]

Common Mistakes:

• Confusing Client with web server modules
• Thinking Client handles file uploads
• Assuming Client manages user authentication

2. Which of the following is the correct way to import the PostgreSQL client in a Deno script?

easy

A. import * as Client from "postgres";

B. import Client from "postgres";

C. const Client = require('postgres');

D. import { Client } from "https://deno.land/x/postgres@v0.17.0/mod.ts";

Solution

1. Step 1: Identify correct import syntax for Deno modules

   Deno uses ES module syntax with full URLs for third-party modules, so import must include the URL and version.

2. Step 2: Check each option

   import { Client } from "https://deno.land/x/postgres@v0.17.0/mod.ts"; uses correct ES module import with URL and version. Options B, C, and D use Node.js style or incomplete imports not valid in Deno.

3. Final Answer:

   import { Client } from "https://deno.land/x/postgres@v0.17.0/mod.ts"; -> Option D

4. Quick Check:

   Deno imports use full URLs = A [OK]

Hint: Deno imports need full URL with version [OK]

Common Mistakes:

• Using Node.js require() syntax
• Omitting the full URL in import
• Using wildcard import incorrectly

3. Given the following Deno code snippet, what will be the output?

import { Client } from "https://deno.land/x/postgres@v0.17.0/mod.ts";

const client = new Client({ user: "user", database: "testdb" });
await client.connect();
const result = await client.queryObject("SELECT 1 AS number");
await client.end();
console.log(result.rows[0].number);

medium

A. undefined

B. 1

C. Error: Connection not established

D. null

Solution

1. Step 1: Analyze the query execution

   The code connects to the database, runs a query selecting the number 1 as 'number', then closes the connection.

2. Step 2: Understand the result object

   The queryObject method returns an object with a rows array. The first row's 'number' field contains 1, so logging it prints 1.

3. Final Answer:

   1 -> Option B

4. Quick Check:

   Query returns 1, printed output = 1 [OK]

Hint: Query returns 1, check rows[0].number for output [OK]

Common Mistakes:

• Forgetting to await client.connect()
• Accessing wrong property of result
• Not closing the client connection

4. Identify the error in this Deno PostgreSQL code snippet:

import { Client } from "https://deno.land/x/postgres@v0.17.0/mod.ts";

const client = new Client({ user: "user", database: "testdb" });
const result = await client.queryObject("SELECT NOW()");
await client.end();
console.log(result.rows);

medium

A. Missing await client.connect() before query

B. Incorrect import statement

C. Using queryObject instead of query

D. Calling client.end() before query

Solution

1. Step 1: Check connection setup

   The code creates a client but never calls await client.connect() before running the query.

2. Step 2: Understand consequences

   Without connecting, the query will fail because the client is not connected to the database.

3. Final Answer:

   Missing await client.connect() before query -> Option A

4. Quick Check:

   Must connect before queries = D [OK]

Hint: Always connect client before running queries [OK]

Common Mistakes:

• Skipping client.connect() call
• Calling client.end() too early
• Confusing queryObject with query

5. You want to insert multiple rows into a PostgreSQL table using Deno's postgres client efficiently. Which approach is best to avoid multiple separate queries and ensure all rows are inserted in one operation?

hard

A. Use a single parameterized query with multiple VALUES tuples

B. Call client.queryObject() multiple times, once per row

C. Run a loop with client.connect() and client.end() for each row

D. Insert rows one by one without parameters

Solution

1. Step 1: Understand batch insertion

   Inserting multiple rows in one query reduces overhead and improves performance.

2. Step 2: Evaluate options

   A uses a single parameterized query with multiple VALUES tuples, which is efficient and safe. B runs multiple queries, increasing overhead. C reconnects repeatedly, which is inefficient. D risks SQL injection and is inefficient.

3. Final Answer:

   Use a single parameterized query with multiple VALUES tuples -> Option A

4. Quick Check:

   Batch insert with one query = A [OK]

Hint: Batch inserts use one query with multiple VALUES [OK]

Common Mistakes:

• Running many separate queries instead of one
• Opening and closing connection repeatedly
• Not using parameterized queries risking SQL injection