Re: WIP: Avoid creation of the free space map for small tables

From: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
To: John Naylor <john(dot)naylor(at)2ndquadrant(dot)com>
Cc: Mithun Cy <mithun(dot)cy(at)enterprisedb(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: WIP: Avoid creation of the free space map for small tables
Date: 2019-01-19 13:05:49
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On Thu, Jan 17, 2019 at 11:13 PM John Naylor
<john(dot)naylor(at)2ndquadrant(dot)com> wrote:
> On Wed, Jan 16, 2019 at 10:35 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > Yes, I think it would be good if you can explain the concept of
> > local-map with the help of this example.
> > Then let's not add a reference to the version number in this case. I
> Okay, done in v14. I kept your spelling of the new macro. One minor
> detail added: use uint8 rather than char for the local map array. This
> seems to be preferred, especially in this file.

I am fine with your change.

Few more comments:
I think we should not allow to create FSM for toast tables as well
till there size reaches HEAP_FSM_CREATION_THRESHOLD. If you try below
test, you can see that FSM will be created for the toast table even if
the size of toast relation is 1 page.

AS $$ select string_agg(chr
(32+(random()*96)::int), '') from generate_series(1,length); $$;

create table tt(c1 int, c2 text);
insert into tt values(1, random_text(2500));
Vacuum tt;

I have fixed this in the attached patch, kindly verify it once and see
if you can add the test for same as well.

-CREATE TABLE test1 (a int, b int);
-INSERT INTO test1 VALUES (16777217, 131584);
+CREATE TABLE test_rel_forks (a
+-- Make sure there are enough blocks in the heap for the FSM to be created.
+INSERT INTO test_rel_forks SELECT g
from generate_series(1,10000) g;

-VACUUM test1; -- set up FSM
+-- set up FSM and VM
+VACUUM test_rel_forks;

This test will create 45 pages instead of 1. I know that to create
FSM, we now need more than 4 pages, but 45 seems to be on the higher
side. I think we should not unnecessarily populate more data if there
is no particular need for it, let's restrict the number of pages to 5
if possible.

-SELECT octet_length(get_raw_page('test1', 'fsm', 1)) AS fsm_1;
- fsm_1
- 8192
-(1 row)
-SELECT octet_length
(get_raw_page('test1', 'vm', 0)) AS vm_0;
+SELECT octet_length(get_raw_page('test_rel_forks', 'fsm', 10)) AS fsm_10;
+ERROR: block number 10 is out of range for relation "test_rel_forks"

Why have you changed the test definition here? Previously test checks
the existing FSM page, but now it tries to access out of range page.

Apart from the above, I have changed one sentence in README.

With Regards,
Amit Kapila.

Attachment Content-Type Size
v15-0001-Avoid-creation-of-the-free-space-map-for-small-heap-.patch application/octet-stream 38.6 KB

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