How Long Does It Take To Create An Index?

A recent tweet asked the following question.

Does anybody know how long it should take to create an index of two integer columns with approximately 110 Million records in the DB? #postgres #postgresql #NotAnDBA

Let's put together an experiment. Create a table with 2 columns populated with 110,000,000 million rows. Each column has a random integer value.

Use the command line psql client that’s included with PostgreSQL to create the index.

To see how long the CREATE INDEX statement will take toggle \timing from psql.

CREATE TABLE t (
    a INTEGER,
    b INTEGER
);

-- Populate 110,000,000 million records
INSERT INTO t(a,b)
SELECT
  (RANDOM() * 1000)::INT,
  (RANDOM() * 1000)::INT
FROM GENERATE_SERIES(1, 110000000) seq;

-- Confirm 110,000,000 records
SELECT COUNT(*) FROM t;
   count
-----------
 110000000

Enable timing to collect the duration of running the CREATE INDEX statement.

-- turn timing on
\timing
Timing is on.

-- Create index "t_a_idx" on table "t" on the "a" column
CREATE INDEX t_a_idx ON t (a);

Time: 52348.022 ms (00:52.348)

Answer

In this test, it took about 52 seconds to create the index.

Test Environment Details

• Tested on a M1 MacBook Air with 16GB RAM.
• The table has no sequences, constraints, or other indexes
• Vacuum was running but was cancelled when the CREATE INDEX started according to the postgresql.log.
• No other queries were running on the table.

This post is intended to be a demonstration of how to conduct this sort of experiment on your own and some of the factors that contribute to the time needed for index creation.

The best way to answer questions like this is to create a test on your own server instance or using a separate disconnected instance made from a snapshot.

In a production system with lice queries to the table, use the CONCURRENTLY keyword when creating indexes, which will possibly double the time to create the index but make it safe to do while other concurrent queries are running.