Re: [GENERAL] [ADMIN] calculating table and index size

On 7 April 2017 at 09:11, Günce Kaya <guncekaya14@gmail.com> wrote:
> Hi again,
>
> Sorry for delay.
>
> Guillaume, I read your answer for first question but It's not clear to me.
> The table has a column and index also use that column. so in that example, I
> think table size and index size should be equal. Why these are not equal?

If I understand correctly, the table only has 1 (integer) column and
all those 1,400,000 rows have the same value?

Then the table has to store each row separately and thus has to store
the same value repeatedly. It also has to store some meta-data, such
as visibility information.

The index on the other hand (assuming a btree index) knows that there
is only a single value in the table and therefore only stores a single
value, but it has to reference each row in the table that contains
that value.

So the table and the index are storing different things, but the total
size of each row/index node for that single integer column is of the
same order of magnitude. That's why they are similar in size.

If you would add another integer column to your table and VACUUM FULL
the table, the table would be about double its size, but the index
would stay the same size.

Regards,
Alban.

> On Wed, Apr 5, 2017 at 1:02 PM, Steven Chang <stevenchang1213@gmail.com>
> wrote:
>>
>> Hello,
>>
>>     try pgstattuple() and pgstatindex() , I think you will figure it out.
>>
>> Steven
>>
>> 2017-04-05 16:56 GMT+08:00 Guillaume Lelarge <guillaume@lelarge.info>:
>>>
>>> Hi,
>>>
>>> 2017-04-05 9:44 GMT+02:00 Günce Kaya <guncekaya14@gmail.com>:
>>>>
>>>> Hi all,
>>>>
>>>> I have some questions about calculating table and index size.
>>>>
>>>> I have a dummy table which has an integer column and its index. The
>>>> table has 1400000 rows and all of rows are same thats value is 20000000.
>>>> Table size is 50MB and index size is 31MB. Why there is too much size
>>>> difference between table and its index? what happen on data files when we
>>>> add index?
>>>>
>>>
>>> You have metadata informations in the table datafiles that you don't have
>>> on the index datafiles. For example, all the system columns for each line.
>>>
>>>>
>>>> Second question is that after created table, table size was 0 byte. I
>>>> inserted a row as 120 then table size was 8192 byte. I inserted five times
>>>> same value to the table and table size is still 8192 bytes. Table size
>>>> changed after inserted lots of rows. Table size was stabile till first few
>>>> hundred rows. why table size didn't change when I inserted lots of rows?
>>>>
>>>
>>> PostgreSQL works with 8KB blocks. When you insert a line, it puts it on a
>>> block, but this block may contain many lines. So your next new lines still
>>> fit in the first block... until it doesn't, and you'll see a new block
>>> coming, making your table datafile grows to 16KB. And so on and so on.
>>>
>>>
>>> --
>>> Guillaume.
>>>   http://blog.guillaume.lelarge.info
>>>   http://www.dalibo.com
>>
>>
>
>
>
> --
> Gunce Kaya
>
> Linkedin - Twitter - Blog

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