From: | Ajay P S <ajayps547(at)gmail(dot)com> |
---|---|
To: | pgsql-hackers(at)lists(dot)postgresql(dot)org |
Subject: | Regarding Plan tree output(Index/Bitmap Scan) |
Date: | 2023-04-12 01:09:41 |
Message-ID: | CAHjjAw17pXwqGQfZeGF24rucDmvW2UqmRoRgtibwQJUfkOA23A@mail.gmail.com |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-hackers |
Hi,
I am trying to understand the Plan tree for select queries. Can you
please help me with the below queries?
1) Why is there a difference in plan tree for these two queries? User
table tidx1 has an index on column 'a' .
2) Why do we do Index scan and not Bitmap Index Scan for catalog tables?
postgres=# explain select * from pg_class where oid=2051;
QUERY PLAN
-------------------------------------------------------------------------------------
Index Scan using pg_class_oid_index on pg_class (cost=0.27..8.29
rows=1 width=265)
Index Cond: (oid = '2051'::oid)
(2 rows)
postgres=# explain select * from tidx1 where a=1;
QUERY PLAN
--------------------------------------------------------------------
Bitmap Heap Scan on tidx1 (cost=4.24..14.91 rows=11 width=8)
Recheck Cond: (a = 1)
-> Bitmap Index Scan on idx1 (cost=0.00..4.24 rows=11 width=0)
Index Cond: (a = 1)
(4 rows)
postgres=# select * from tidx1;
a | b
---+---
1 | 2
2 | 2
3 | 2
4 | 2
5 | 2
(5 rows)
Best,
Aj
From | Date | Subject | |
---|---|---|---|
Next Message | Stephen Frost | 2023-04-12 01:18:08 | Re: longfin missing gssapi_ext.h |
Previous Message | Michael Paquier | 2023-04-12 01:07:08 | Re: Add LZ4 compression in pg_dump |