Thread: a question about oddities in the data directory

Apologies if I asked this question before.

I’m a newcomer to Postgres, having migrated from Mysql.  Information about tables seems harder to get at in Postgres.
Thatdata directory on my machine is suspiciously large—well over 100 GB.  The directory Postgres/var-9.5/base/ contains
anumber of subdirectories with Arabic numerals. Directory 16385 has a subdirectory 17463 with a size of 1.07 GB. But
thereare also 17 subdirectories with names from 17463.1 to 17.463.17.  There are also other entries with similar forms
ofduplication and suspiciously identical file sizes of 1.07GB.
 

Is this normal behavior?  Where in the postgres documentation do I read up on this? Postgres strikes me as superior to
MySQl,especially with regard to string functions and regular expressions, but it’s harder to look under the hood. How,
forinstance, do I figure out what number corresponds to the table that I know as ‘earlyprinttuples
 

With thanks in advance for any advice

Martin Mueller
Professor emeritus of English and Classics

Hi,

I think you need to read this:

https://www.postgresql.org/docs/current/static/storage-file-layout.html

On 11/27/2017 04:19 PM, Martin Mueller wrote:
> Apologies if I asked this question before.
> 
> I’m a newcomer to Postgres, having migrated from Mysql.  Information
> about tables seems harder to get at in Postgres. That data directory
> on my machine is suspiciously large—well over 100 GB.  The directory
> Postgres/var-9.5/base/ contains a number of subdirectories with
> Arabic numerals.

Each of those directories stores data for a single database. Each object
(including databases) has a 32-bit identifier, mapping it to object on
the filesystem.

databases: SELECT oid, datname FROM pg_database;
tables etc.: SELECT relfilenode, relname FROM pg_class;

> Directory 16385 has a subdirectory 17463 with a size of 1.07 GB.
That's not a subdirectory, but a datafile segment.

> But there are also 17 subdirectories with names from 17463.1 to
> 17.463.17.  There are also other entries with similar forms of
> duplication and suspiciously identical file sizes of 1.07GB.
> 

Again, those are files, not subdirectories. Large datafiles are split
into 1GB segments, so for example 10GB table with relfilenode 17463 will
be stored in files 17463, 17463.1, 17463.2, ..., 17463.9

> Is this normal behavior?  Where in the postgres documentation do I
> read up on this? Postgres strikes me as superior to MySQl, especially
> with regard to string functions and regular expressions, but it’s
> harder to look under the hood.

https://www.postgresql.org/docs/current/static/storage-file-layout.html

> How, for instance, do I figure out what number corresponds to the
> table that I know as ‘earlyprinttuples
> 

SELECT relfilenode FROM pg_class WHERE relname = 'earlyprinttuples';


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

On Mon, 2017-11-27 at 15:19 +0000, Martin Mueller wrote:
>  Apologies if I asked this question before.
> Is this normal behavior? 

Yes.

>  Where in the postgres documentation do I read up on this? 

Start here - https://www.postgresql.org/docs/10/static/admin.html

> string functions and regular expressions, but it’s harder to look
> under the hood.

I would strongly recommend staying out from under the hood.  PostgreSQL
has really great tools from visualizing what is going on in the
database(s);  use the tools.

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