Cluster

Cluster allows you to sort a table by a specific index:

Imagine this table:

db= TRUNCATE sale_fact;
TRUNCATE TABLE
 
-- Insert rows without sorting
db= INSERT INTO sale_fact (username, sold_at)
db- SELECT
db-     md5(random()::text) AS username,
db-     '2020-01-01'::date + (interval '1 day') * round(random() * 365 * 2) AS sold_at
db- FROM
db-     generate_series(1, 100000)
INSERT 0 100000
 
db= ANALYZE sale_fact;
ANALYZE
 
db= SELECT tablename, attname, correlation
db- FROM pg_stats
db- WHERE tablename = 'sale_fact';
 
  tablename | attname  |  correlation
-----------+-----------+----------------
 sale_fact | sold_at   | -5.9702674e-05
 sale_fact | id        |              1
 sale_fact | username  |    0.010033822

We have 0 correlation on sold_at, so if we query using a WHERE on sold_at, we’ll get a Bitmap Heap Scan, which will be much slower than an Index Scan.

Let’s cluster on our sold_at index.

db= CLUSTER sale_fact USING sale_fact_sold_at_ix;
CLUSTER
 
db= ANALYZE sale_fact;
ANALYZE
 
db= SELECT tablename, attname, correlation
db- FROM pg_stats
db- WHERE tablename = 'sale_fact';
 
 tablename | attname  | correlation
-----------+----------+--------------
 sale_fact | sold_at  |            1
 sale_fact | id       | -0.002239401
 sale_fact | username |  0.013389298

Clustering affects the correlatio of other columns, like id, which went from being a correlation of 1 to 0. Try to avoid this by inserting data sorted, by inserting the data with an ORDER BY.

Compare that table above with a correlation table using an ORDER BY on an insert:

db= SELECT tablename, attname, correlation
db- FROM pg_stats
db= WHERE tablename = 'sale_fact';
 tablename | attname  |  correlation
-----------+----------+----------------
 sale_fact | id       |              1
 sale_fact | username | -0.00041992788
 sale_fact | sold_at  |              1

Queries that use the id index and sold_at index will be much faster, whereas with a cluster, only queries on sold_at will be fast.