Re: [SQL] generic crosstab ?

[Samuel Gendler]

On Tue, Apr 24, 2012 at 1:01 PM, Andreas maps...@gmx.net wrote:

 Hi,

 is there a generic solution to dump the result of a query as a crosstab,
 when I can't know how many columns I will need?

 E.g. I get something like this:

 id, x
 1,  a
 1,  b
 1,  c
 2,  l
 2,  m



Yes.  You can provide a query which returns the columns to the version of
the crosstab function which looks like this:

crosstab(text source_sql, text category_sql)
It does exactly what you are looking for. The second query returns the set
of values that act as columns in the final result (the pivot for each row
in the result returned by the first query).  This allows the function to
correctly insert a null for any column for which there is no row in the
first query results.

Re: [SQL] generic crosstab ?

[Andreas]

Am 24.04.2012 22:08, schrieb Samuel Gendler:



On Tue, Apr 24, 2012 at 1:01 PM, Andreas maps...@gmx.net 
mailto:maps...@gmx.net wrote:


Hi,

is there a generic solution to dump the result of a query as a
crosstab, when I can't know how many columns I will need?

E.g. I get something like this:

id, x
1,  a
1,  b
1,  c
2,  l
2,  m



Yes.  You can provide a query which returns the columns to the version 
of the crosstab function which looks like this:


|crosstab(text source_sql, text category_sql)|

It does exactly what you are looking for. The second query returns the 
set of values that act as columns in the final result (the pivot for 
each row in the result returned by the first query).  This allows the 
function to correctly insert a null for any column for which there is 
no row in the first query results.





I got stuck with an error that translates to Materialisation mode is 
needed but is not allowed in this context.

I couldn't figure out what this materialisation mode is, yet.

Could you please have a look at my query sample?
Both queries work for themselves but crosstab() fails.   :(
I checked and there are never more than 20 child_ids per parent_id so 
there should be enough columns.


select
crosstab (
$$
select
parent_idas  row_name,
'x' || row_number() over ( partition by parent_id order by 
child_id )  as  category,

child_id  as  value
from
children
order by 1
$$,
$$
select 'x' || generate_series(1, 20) as  cat  order by 1
$$
);

Re: [SQL] generic crosstab ?

[David Johnston]

From: pgsql-sql-ow...@postgresql.org [mailto:pgsql-sql-ow...@postgresql.org]
On Behalf Of Andreas
Sent: Tuesday, April 24, 2012 5:35 PM
To: Samuel Gendler
Cc: pgsql-sql@postgresql.org
Subject: Re: [SQL] generic crosstab ?

 

Am 24.04.2012 22:08, schrieb Samuel Gendler: 

 

On Tue, Apr 24, 2012 at 1:01 PM, Andreas maps...@gmx.net wrote:

Hi,

is there a generic solution to dump the result of a query as a crosstab,
when I can't know how many columns I will need?

E.g. I get something like this:

id, x
1,  a
1,  b
1,  c
2,  l
2,  m



 

Yes.  You can provide a query which returns the columns to the version of
the crosstab function which looks like this:

 


crosstab(text source_sql, text category_sql)

It does exactly what you are looking for. The second query returns the set
of values that act as columns in the final result (the pivot for each row in
the result returned by the first query).  This allows the function to
correctly insert a null for any column for which there is no row in the
first query results.

 

 


I got stuck with an error that translates to Materialisation mode is needed
but is not allowed in this context.
I couldn't figure out what this materialisation mode is, yet.

Could you please have a look at my query sample?
Both queries work for themselves but crosstab() fails.   :(
I checked and there are never more than 20 child_ids per parent_id so there
should be enough columns.

select
crosstab (
$$
select
parent_idas  row_name,
'x' || row_number() over ( partition by parent_id order by child_id
)  as  category,
child_id  as  value
from
children
order by 1
$$,
$$
select 'x' || generate_series(1, 20) as  cat  order by 1
$$
);

 

 

You must specify the output record structure:

 

SELECT crosstab(text, text) AS ( col1_name col1_type [, colN_name
colN_type]* )

 

See: http://www.postgresql.org/docs/9.0/interactive/tablefunc.html for
official usage and examples

 

Whether this relates to the materialization node message you are receiving
I have no idea.

 

Dave

 

Re: [SQL] generic crosstab ?

[Joe Conway]

On 04/24/2012 02:42 PM, David Johnston wrote:
 You must specify the output record structure:
 
 SELECT crosstab(text, text) AS ( col1_name col1_type [, colN_name
 colN_type]* )
 
 Whether this relates to the “materialization node” message you are
 receiving I have no idea.

The error is because you are selecting from a set returning function in
the target list rather than the from clause. It should be more like:

SELECT * FROM crosstab(text, text) AS ( col1_name col1_type [, colN_name
 colN_type]* )

HTH,

Joe

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credativ LLC: http://www.credativ.us
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Training, Service, Consulting,  24x7 Support

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Re: [SQL] generic crosstab ?

[Andreas]

Am 25.04.2012 00:04, schrieb Joe Conway:

On 04/24/2012 02:42 PM, David Johnston wrote:

You must specify the output record structure:

SELECT crosstab(text, text) AS ( col1_name col1_type [, colN_name
colN_type]* )

Whether this relates to the “materialization node” message you are
receiving I have no idea.

The error is because you are selecting from a set returning function in
the target list rather than the from clause. It should be more like:

SELECT * FROM crosstab(text, text) AS ( col1_name col1_type [, colN_name

colN_type]* )




OK now i get at least some result.
But
1) I need to know how many categories will apear to construct the target 
list. This is a wee bit of a problem as this number is actually dynamic.


2) There are some rows in the resulting list with empty columns within 
the row.
When I execute the first query for a parent ID that has gaps in the 
crosstab I see it shows no gaps in the categories when called outside 
crosstab().
E.g. it dumps  x1, x2, x3, x4, x5 when called seperately but crosstab() 
shows

x1, x2, null, null, x5, null, x6, x7

How does this make sense ?


Thanks for the answers so far   :)


select  *
from
crosstab (
$$
select
parent_idas  row_name,
'x' || row_number() over ( partition by parent_id order by 
child_id )  as  category,

child_id  as  value
from
children
order by 1
$$,
$$
select 'x' || generate_series(1, 15) as  category  order by 1
$$
)
as result (
row_nameinteger,
x1  integer,
x2  integer,
x3  integer,
x4  integer,
x5  integer,
x6  integer,
x7  integer,
x8  integer,
x9  integer,
x10 integer,
x11 integer,
x12 integer,
x13 integer,
x14 integer,
x15 integer
)

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Re: [SQL] generic crosstab ?

[Pavel Stehule]

Hello

try to use cursors

http://okbob.blogspot.com/2008/08/using-cursors-for-generating-cross.html

Regards

Pavel Stehule

2012/4/24 Andreas maps...@gmx.net:
 Hi,

 is there a generic solution to dump the result of a query as a crosstab,
 when I can't know how many columns I will need?

 E.g. I get something like this:

 id, x
 1,  a
 1,  b
 1,  c
 2,  l
 2,  m


 and I'd like to see it as:

 id,  x1,  x2,  x3,  .  xn
 1,   a,   b,   c,    null,  null
 2,   l,    m,  

 I fear the problem is I dont know n.

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Re: [SQL] generic crosstab ?

[Samuel Gendler]

On Tue, Apr 24, 2012 at 3:37 PM, Andreas maps...@gmx.net wrote:

 Am 25.04.2012 00:04, schrieb Joe Conway:

  On 04/24/2012 02:42 PM, David Johnston wrote:

 You must specify the output record structure:

 SELECT crosstab(text, text) AS ( col1_name col1_type [, colN_name
 colN_type]* )

 Whether this relates to the “materialization node” message you are
 receiving I have no idea.

 The error is because you are selecting from a set returning function in
 the target list rather than the from clause. It should be more like:

 SELECT * FROM crosstab(text, text) AS ( col1_name col1_type [, colN_name

 colN_type]* )



 OK now i get at least some result.
 But
 1) I need to know how many categories will apear to construct the target
 list. This is a wee bit of a problem as this number is actually dynamic.

 2) There are some rows in the resulting list with empty columns within the
 row.
 When I execute the first query for a parent ID that has gaps in the
 crosstab I see it shows no gaps in the categories when called outside
 crosstab().
 E.g. it dumps  x1, x2, x3, x4, x5 when called seperately but crosstab()
 shows
 x1, x2, null, null, x5, null, x6, x7

 How does this make sense ?


I believe that the crosstab function is fairly naive about constructing the
crosstab and requires that each row in the first query be in row-order for
the crosstab output.  It is possible that it even requires the rows in the
first query to be in column order in the crosstba output.

In other words, if the first query returns results like this:

row1, col3, value1
row1, col1, value2
row2, col1, value3
row2, col3, value4
row1, col2, value5

I believe that the last row1 entry will be dropped, or else maybe you'll
get two rows in the crosstab output, each representing row1, and I am
hypothesizing (without re-reading the documentation) that the fact that
col3 comes before col1 in row1 will also result in a hole in the output.

Basically, your first query should include order by 1,2 rather than just
order by 1

Have you gone through the documentation for the crosstab functions?  I
believe that will answer most of your questions.  It has been a long time
since I've read them, but I remember them being fairly complete.

As for your question about knowing how many columns you will have, I've
always solved that by opening a serialized transaction and running a query
to get the possible set of columns, then issue the sql statement that
includes the crosstab(sql,sql) call, using the first query results to build
up the return result type.  Depending on the nature of your data set, it is
possible you can forgo the serialized transaction, since there may be
little to no risk of new column names appearing between the two queries.
 You could also just structure the queries in the second statement so that
it is guaranteed to return only the results returned by your initial query
- include a where clause which is guaranteed to return only rows that match
the columns returned by your first query.  You may wind up missing a value
that was added between the first query and the second query, but odds are
good that if your data is that volatile, it doesn't matter if you miss a
value which happened to be inserted in those few milliseconds.

--sam