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Re: Converting postgresql.conf parameters to kilobytes

From: Shridhar Daithankar <shridhar(at)frodo(dot)hserus(dot)net>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Converting postgresql.conf parameters to kilobytes
Date: 2004-06-01 12:31:56
Message-ID: 200406011801.56840.shridhar@frodo.hserus.net (view raw or flat)
Thread:
Lists: pgsql-hackers
On Tuesday 01 June 2004 14:12, Shridhar Daithankar wrote:
> Actually I need to find out few more things about it. It is not as simple
> as adding a assign_hook. When I tried to initdb with changes, it demanded
> 64MB of shared buffers which I (now) think that somewhere NBuffers are used
> before postgresql.conf is parsed. So 8192*8000=64MB. But this is just
> guesswork. Haven't looked in it there.

Found  it. Following is the code that is causing problem.

guc.c:2998
-----------
				if (conf->assign_hook)
					if (!(*conf->assign_hook) (newval, changeVal, source))
					{
						ereport(elevel,
								(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
								 errmsg("invalid value for parameter \"%s\": %d",
										name, newval)));
						return false;
					}

				if (changeVal || makeDefault)
				{
					if (changeVal)
					{
						*conf->variable = newval;
						conf->gen.source = source;
					}
-----------

So even if assign_hook is executed, the value of variable is overwritten in 
next step which nullifies any factoring/change in value done in assign hook.

I find this as a convention at many other place at guc.c. Call assign_hook and 
the overwrite the value. So is assign_hook called only to validate the value?  
How do I modify the value of the variable without getting specific?

I tried 

if (changeVal && !(conf->assign_hook))

and it worked. However that is just for int variables. I am not sure if that 
is a design decision. What should I do?

Regards,
 Shridhar

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Subject: Re: SELECT * FROM <table> LIMIT 1; is really slow
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