Re: [HACKERS] One-shot expanded output in psql using \G

[D'Arcy Cain]

On 2017-01-27 10:05 AM, David Fetter wrote:

On Fri, Jan 27, 2017 at 02:27:37PM +0100, Christoph Berg wrote:

I frequently find myself in the situation that I want the "\x"
expanded output mode activated just for one query. There's little
wrong with typing "\x" and re-executing the query in that case, but
then I'm always annoyed that the expanded output is still active for
the next query after that.


+1

Your situation is familiar to me, and likely common among heavy users
of psql.


I am a pretty heavy user of psql but I don't think that that would be so 
helpful.  I assume you mean a new option, let's call it "\X" the causes 
the next query to be expanded.  I type "\X" then a query.  I realize 
that I made a mistake and have to redo the query so I have to type "\X" 
again.  If I forget then I have to run the query yet again.


What would be useful for me is a control that causes every query to be 
expanded if it returns exactly one row.  Now I can do this:


SELECT * FROM table WHERE table_status = 1; % get a short list
SELECT * FROM table WHERE table_id = 123; % drill down to one ID

The first would give me a normal list and the second would give me an 
expanded look at the one I want to see in detail.


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Re: [HACKERS] COMMUTATOR doesn't seem to work

[D'Arcy Cain]

On 12-06-23 12:17 AM, Tom Lane wrote:

D'Arcy Cain da...@druid.net writes:

On 12-06-22 07:09 PM, Robert Haas wrote:

I think DirectionFunctionCall2 is what you want.



Can you elaborate?  I could not find a single hit in Google or the
documentation search on the PG site and it does not appear anywhere
in the source distribution.


He meant DirectFunctionCall2


Still nothing in the documentation.  At least the search box doesn't
find it.

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Re: [HACKERS] COMMUTATOR doesn't seem to work

[D'Arcy Cain]

On 12-06-23 08:21 AM, Dickson S. Guedes wrote:

Still nothing in the documentation.  At least the search box doesn't
find it.


Try in the search box of postgres doxygen documentation [1]..


That's source, not documentation.  I already found it in the actual
source files but that's not the same thing.  For one thing, if it
isn't documented then it may not be meant to be exposed to the API.
For another, without documentation we can't tell what is guaranteed
to work and what is an implementation detail.

I know that I can fiddle about and try stuff until it works but I
don't work for Microsoft.

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Re: [HACKERS] COMMUTATOR doesn't seem to work

[D'Arcy Cain]

On 12-06-22 07:11 AM, Florian Pflug wrote:

On Jun22, 2012, at 06:32 , D'Arcy Cain wrote:

So I have my type working now but I had to create a new C function
that take the opposite argument order.  Seems redundant but I could
not see a better way.


There isn't. Postgres itself contains a huge number of such functions,
e.g. for every *lt() (less-than) there's a *gt() (greater-than).


Right but that's not the same thing.  Assuming you meant lt/gte and
lte/gt those still are not self-commutating (SC).  For example, '='
on two ints is SC.  The issue here is that the operator is SC but
the args are different types.  It would be nice if there was a way
to automatically generate code that reverses arguments.  Maybe such
a thing belongs in the CREATE FUNCTION command.

Or, I guess this works and keeps from creating a second C function:

CREATE OR REPLACE FUNCTION eq(chkpass, text)
RETURNS bool
STRICT
AS 'chkpass.so', 'chkpass_eq'
LANGUAGE 'c'
RETURNS NULL ON NULL INPUT;

CREATE OR REPLACE FUNCTION eq(text, chkpass)
RETURNS bool
AS 'select eq($2, $1);'
LANGUAGE SQL
RETURNS NULL ON NULL INPUT;


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Re: [HACKERS] COMMUTATOR doesn't seem to work

[D'Arcy Cain]

On 12-06-22 11:36 AM, Tom Lane wrote:

D'Arcy Cainda...@druid.net  writes:
The thing is that either of those approaches is hugely more expensive
than just providing a second C function.  It costs probably thousands
of cycles to inline that SQL function, each time it's used in a query.


I assumed itwould be more expensive but didn't know it would be that
much more.


I doubt that an auto reverse the arguments facility would be very
much cheaper.  You could maybe argue that the aggregated maintenance
and space costs of all the commutator-pair functions are enough to
justify having some such solution instead, but I'm doubtful --- and
even if true, getting from here to there would be painful.


And it would only apply to a very specific type of function.

The other idea I had was to just have the second C function call the
first but that didn't work.  Here is what I tried.

PG_FUNCTION_INFO_V1(chkpass_eq);
Datum
chkpass_eq(PG_FUNCTION_ARGS)
{
chkpass*a1 = (chkpass *) PG_GETARG_POINTER(0);
text   *a2 = (text *) PG_GETARG_TEXT_P(1);
charstr[9];

strlcpy(str, a2-vl_dat, sizeof(str));
PG_RETURN_BOOL(strcmp(a1-password, crypt(str, a1-password)) == 0);
}

PG_FUNCTION_INFO_V1(chkpass_eq2);
Datum
chkpass_eq2(PG_FUNCTION_ARGS)
{
return chkpass_eq(PG_GETARG_POINTER(1), PG_GETARG_TEXT_P(0));
}

Now in this specific case the function is trivial and writing it twice
is no big deal but in general I hate writing the same code twice.  I
suppose I could extract the actual operation out to a third function
and call it from the others.  I may do that anyway just for the value
of the example.  Or is there a way to do what I tried above?

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Re: [HACKERS] COMMUTATOR doesn't seem to work

[D'Arcy Cain]

On 12-06-22 07:09 PM, Robert Haas wrote:

I think DirectionFunctionCall2 is what you want.


Can you elaborate?  I could not find a single hit in Google or the
documentation search on the PG site and it does not appear anywhere
in the source distribution.

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[HACKERS] COMMUTATOR doesn't seem to work

[D'Arcy Cain]

Maybe I am using it wrong but I get no error message when I use it.  I
have a type called chkpass (a version is in the additional supplied
modules) and I create the equality operator like this:

CREATE OPERATOR = (
PROCEDURE = eq,
LEFTARG = chkpass,
RIGHTARG = text,
COMMUTATOR = =,
NEGATOR = 
);

This works;

cosmostest=# SELECT 'aaa'::chkpass = 'aaa';
 ?column?
--
 t
(1 row)

But...

cosmostest=# SELECT 'aaa' = 'aaa'::chkpass;
ERROR:  operator is only a shell: text = chkpass
LINE 1: SELECT 'aaa' = 'aaa'::chkpass;

When I look at the operators I see why, sort of...

 public | =| chkpass   | text   | boolean |
 public | =| text  | chkpass| -   |

So while it created the operator it didn't set a return type.

I don't know if this is a new issue or I simply got lucky and never
tried the opposite test before but I see this in 9.0.4 and 9.1.3.
Am I using the command improperly?

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Re: [HACKERS] COMMUTATOR doesn't seem to work

[D'Arcy Cain]

On 12-06-21 12:18 PM, Tom Lane wrote:

Did you actually create a text = chkpass function and operator?
This declaration merely promises that you will provide one eventually.
The system does not have the ability to make one for you.


I guess I am missing the point of COMMUTATOR then.  The docs say When
you are defining a self-commutative operator, you just do it.  It
seems you need to do more than just do it.  As far as I can tell I
will need to add another C function and another CREATE OPERATOR.
Then I can create an operator that takes args (text, chkpass) instead
of (chkpass, text).  What is the COMMUTATOR for then?  Is it just a
hint to the planner?

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Re: [HACKERS] COMMUTATOR doesn't seem to work

[D'Arcy Cain]

On 12-06-22 12:22 AM, Tom Lane wrote:

Um, an operator with different types on left and right cannot be its own
commutator.


Understood.  I completely misunderstood the purpose of COMMUTATOR.  I
thought that it was telling the system that the procedure could be
called with the arguments reversed if it could not find a specific
procedure.  I see now what it is for.

So I have my type working now but I had to create a new C function
that take the opposite argument order.  Seems redundant but I could
not see a better way.

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