INDEX plan

drop table if exists tb;
create table tb (a int, b bigint);
insert into tb select n, n from generate_series(1, 100000) as n;
ANALYZE tb;

sequence scan

explain select * from tb where a = 5432;
                      QUERY PLAN
------------------------------------------------------
 Seq Scan on tb  (cost=0.00..1791.00 rows=1 width=12)
   Filter: (a = 5432)

index scan

create index idx on tb(a);
ANALYZE tb;

explain select * from tb where a = 5432;
                          QUERY PLAN
---------------------------------------------------------------
 Index Scan using idx on tb  (cost=0.29..8.31 rows=1 width=12)
   Index Cond: (a = 5432)

why width = 12?

select attname, avg_width from pg_stats where tablename='tb';
 attname | avg_width
---------+-----------
 a       |         4
 b       |         8

Index Only Scan

explain select a from tb where a = 5432;
                            QUERY PLAN
-------------------------------------------------------------------
 Index Only Scan using idx on tb  (cost=0.29..4.31 rows=1 width=4)
   Index Cond: (a = 5432)

Bitmap Index Scan

explain select * from tb where a = 5000 or a = 8000;
                               QUERY PLAN
------------------------------------------------------------------------
 Bitmap Heap Scan on tb  (cost=8.60..16.27 rows=2 width=12)
   Recheck Cond: ((a = 5000) OR (a = 8000))
   ->  BitmapOr  (cost=8.60..8.60 rows=2 width=0)
         ->  Bitmap Index Scan on idx  (cost=0.00..4.30 rows=1 width=0)
               Index Cond: (a = 5000)
         ->  Bitmap Index Scan on idx  (cost=0.00..4.30 rows=1 width=0)
               Index Cond: (a = 8000)

explain select * from tb where a in (5000, 8000);
                           QUERY PLAN
----------------------------------------------------------------
 Index Scan using idx on tb  (cost=0.29..12.62 rows=2 width=12)
   Index Cond: (a = ANY ('{5000,8000}'::integer[]))

why different with a in(5000, 8000)?

B-Tree 索引的多键下跳（Multi-Index Scan / Multi-scan） | ScalarArrayOpExpr Index Optimization