How to create a schema-bound function that counts nodes in an xml and then be able to persist this result in a column

Question

I have an xml column and I want to persist a node count in it in an adjacent column.

For example, in an xml given below, I have 3 nodes a and so I want to output it.

The problem that I have, however, is that I can't manage to create a generic schema-bound function that would take @xml and @nodeName so as to return the count of a specified node.

create function dbo.fnXmlCount_a (@xml xml) 
returns int with schemabinding
as begin
return (@xml.value('count(//a)', 'int'))
end

declare @xml xml;
set @xml = '
<r>
 <a></a>
 <b></b>
 <a></a>
 <c>
  <a></a> 
 </c>
</r>'

select dbo.fnXmlCount_a(@xml) aCount

Result of the last operation is:

aCount
3

And then I create a column on that table that has this xml field like so:

alter table [XmlTable] add column [aCount] as (dbo.fnXmlCount_a([xml])) persisted

But what I really would like to do is this:

alter table [XmlTable] add column [aCount] as (dbo.fnXmlNodeCount([xml], 'a')) persisted

Thanks!

Update:

We all know that // is really slow, so it would be great if you could figure out how to pass this path to search in in another variable; by default, this searching path would be //.

Answer 1

Here you go:

create function dbo.fnXmlCount (@xml xml, @name nvarchar(max))  
returns int with schemabinding 
as begin 
declare @count int
select @count = count(*)
from @xml.nodes('//*') as t(c)
where t.c.value('local-name(.)', 'nvarchar(max)') = @name
return @count
end 

Answer 2

I have come up with a little better solution but the approach is similar to that of Daniel Renshaw.

create function [dbo].[fnXmlCount] (@xml xml, @nodeName nvarchar(max))  
returns int with schemabinding 
as begin 
return @xml.value('count(//*[local-name() = sql:variable("@nodeName")])', 'int')
end 

However, there's still no solution as to how to pass the path to nodes being counted.

And it takes about 20 seconds to count nodes on my xml table.