Introduction to res.fetchall()

res.fetchall() is a method used to retrieve all the rows of a Python Database API (DB-API) query result set and return them as a list of tuples. The method is called on a cursor object created after executing a SELECT statement on a database connection using the execute() method.

The returned list will contain one tuple for each row in the result set. Each tuple will contain the values for each column specified in the SELECT statement, in the order specified. The data types of the values in each tuple will correspond to the data types of the columns in the database table.

It is important to note that using res.fetchall() can potentially lead to performance issues if there is a large amount of data being retrieved, as all of the rows are loaded into memory at once. Additionally, if the result set is empty, an empty list will be returned.

Here is an example of how res.fetchall() can be used:

import psycopg2

# Establish a connection to a PostgreSQL database
conn = psycopg2.connect(database="example_db", user="username", password="password", host="localhost", port="5432")

# Create a cursor object
cur = conn.cursor()

# Execute a SELECT statement
cur.execute("SELECT * FROM example_table")

# Retrieve all the rows of the result set as a list of tuples
result_set = cur.fetchall()

# Iterate through the list of tuples and print the values
for row in result_set:
    print(row)

# Close the cursor and connection
cur.close()
conn.close()

In this example, res.fetchall() retrieves all the rows of a SELECT statement and stores them in the result_set variable. The for loop then iterates through the list of tuples and prints the values. Finally, the cursor and connection are closed.

Overall, res.fetchall() is a useful method for retrieving all the rows of a query result set and working with them as a list of tuples. However, it should be used with caution when dealing with large amounts of data.