Re: EXPLAIN BUFFERS

From: Itagaki Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>
To: Euler Taveira de Oliveira <euler(at)timbira(dot)com>
Cc: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Pgsql Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: EXPLAIN BUFFERS
Date: 2009-12-08 02:58:57
Message-ID: 20091208115857.482E.52131E4D@oss.ntt.co.jp
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Here is an updated patch per discussion.

* Counters are accumulative. They contain I/Os by child nodes.
* Text format shows all counters.
* Add "shared_" prefix to variables representing shared buffers/blocks.

Euler Taveira de Oliveira <euler(at)timbira(dot)com> wrote:

> Itagaki Takahiro escreveu:
> > I think the current output is enough and useful in normal use.
> > We can use XML or JSON format for more details.
> >
> I don't think it is a good idea to have different information in different
> formats. I'm with Robert; *don't* do that.

I'm afraid of the human-unreadability of the text format, that is discussed
in the YAML format thread. ...but I found we say the following in the docs.

XML or JSON output contains the same information as the text output format
http://developer.postgresql.org/pgdocs/postgres/sql-explain.html

Obviously I should not hide any information only in the text format.
The new output will be: (in one line)
Shared Blocks: (hit=2 read=1641 written=0) Local Blocks: (hit=0 read=0 written=0) Temp Blocks: (read=1443 written=1443)

> There are nodes that don't read or write blocks.

This will be impossible now because we re-defined the meaning of counters
as "accumulated number of I/O". Even if the node never read or write blocks,
it might contain some child nodes with I/O.

Regards,
---
ITAGAKI Takahiro
NTT Open Source Software Center

Attachment Content-Type Size
explain_buffers_20091208.patch application/octet-stream 20.6 KB

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