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Re: Query progress indication - an implementation

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: Josh Berkus <josh(at)agliodbs(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Scara Maccai <m_lists(at)yahoo(dot)it>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Query progress indication - an implementation
Date: 2009-06-29 22:49:54
Message-ID: 1282.1246315794@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> On Mon, 2009-06-29 at 14:07 -0400, Tom Lane wrote:
>> I think this is pretty much nonsense --- most queries run all their plan
>> nodes concurrently to some extent.  You can't usefully say that a query
>> is "on" some node, nor measure progress by whether some node is "done".

> The requirement is not nonsense, even if the detail was slightly off.

I was applying the word "nonsense" to the proposed implementation,
not the desire to have query progress indications ...

> We can regard plans as acting in phases with each blocking node
> separating the plan. We know which nodes those are, so we can report
> that.

[ shrug... ] You can regard them that way, but you won't get
particularly helpful results for a large fraction of real queries.
The system is generally set up to prefer "streaming" evaluation
as much as it can.  Even in nominally blocking nodes like Sort and Hash,
there are operational modes that look more like streaming, or at least
chunking.

			regards, tom lane

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