One example: PostgreSQL doesn't use index with search condition on not balanced column

README.md

How to solve this problem

• change random_page_cost

How to run

Setup

1. run postgres

docker run --rm -p 5432:5432 --name postgres -e POSTGRES_PASSWORD=pass -d postgres:latest

2. generate table records with gen_table1_records.py

python gen_ps_table_record.py > sample.csv

3. create table and import csv data

psql -h localhost -p 5432 -W -U postgres

postgres=# CREATE TABLE test1 (id integer primary key, group_id integer, name text);
postgres=# CREATE INDEX idx_group_id ON test1 USING btree (group_id);
postgres=# \copy test1 (id, group_id, name) from 'sample.csv' with csv;

EXPLAIN query

1. seasrch with 'general' values

postgres=# EXPLAIN ANALYZE select * from test1 where group_id = 10;
                                                      QUERY PLAN                                                      
----------------------------------------------------------------------------------------------------------------------
 Index Scan using idx_group_id on test1  (cost=0.42..9.31 rows=33 width=14) (actual time=0.039..0.039 rows=1 loops=1)
   Index Cond: (group_id = 10)
 Planning time: 0.222 ms
 Execution time: 0.053 ms

2. seasrch with 'rare' values

postgres=# EXPLAIN ANALYZE select * from test1 where group_id = 5;
                                                   QUERY PLAN                                                    
-----------------------------------------------------------------------------------------------------------------
 Seq Scan on test1  (cost=0.00..17907.25 rows=999900 width=14) (actual time=0.010..109.217 rows=1000001 loops=1)
   Filter: (group_id = 5)
   Rows Removed by Filter: 99
 Planning time: 0.063 ms
 Execution time: 141.340 ms

gen_table1_records.py

special_group_id = 5
other_records = 100
for x in range(other_records):
    print(f"{x},{x},{x}")


for x in range(1000000):
    id = other_records + x
    print(f"{id},{special_group_id},{id}")