Обсуждение: xpath() subquery for empty array

The function xpath evaluates the XPath expression xpath against the XML value xml. It returns an array of XML values corresponding to the node set produced by the XPath expression.

SELECT x
FROM (SELECT xpath('//entry[contains(p, ''searchtext'')]/@*', docxml)
AS x FROM docs) AS y WHERE x <> [test for empty array?????????]

Roy Walter wrote:
> In postgres 8.4 When running xpath() queries it seems that empty results
> are always returned. So if I query a table containing 1000 XML documents
> a 1000 rows will always be fetched even if the xpath() element of the
> query only matches 10 documents.
>
> The documentation states:
>
>     The function |xpath| evaluates the XPath expression /xpath/ against
>     the XML value /xml/. It returns an array of XML values corresponding
>     to the node set produced by the XPath expression.
>
> Yet it seems that empty node sets (arrays) are also returned. So, my
> thought was to run a subquery to eliminate empty arrays, e.g.:
>
>     SELECT x
>     FROM (SELECT xpath('//entry[contains(p, ''searchtext'')]/@*', docxml)
>     AS x FROM docs) AS y WHERE x <> [test for empty array?????????]
>
> How do I test for an empty array in postgres?
>
> Thanks in advance,
> Roy

WHERE x != array[]::xml[]

Roy Walter wrote:

In postgres 8.4 When running xpath() queries it seems that empty results are always returned. So if I query a table containing 1000 XML documents a 1000 rows will always be fetched even if the xpath() element of the query only matches 10 documents.

The documentation states:

    The function |xpath| evaluates the XPath expression /xpath/ against
    the XML value /xml/. It returns an array of XML values corresponding
    to the node set produced by the XPath expression.

Yet it seems that empty node sets (arrays) are also returned. So, my thought was to run a subquery to eliminate empty arrays, e.g.:

    SELECT x
    FROM (SELECT xpath('//entry[contains(p, ''searchtext'')]/@*', docxml)
    AS x FROM docs) AS y WHERE x <> [test for empty array?????????]

How do I test for an empty array in postgres?

Thanks in advance,
Roy

WHERE x != array[]::xml[]

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ERROR:  syntax error at or near "]"
LINE 3: AS x FROM docs) AS y WHERE x != array[]::xml[]
                                              ^

On Sun, Jul 12, 2009 at 06:41:57PM +0100, Roy Walter wrote:
> Scott Bailey wrote:
> >Roy Walter wrote:
> >>How do I test for an empty array in postgres?
> >
> >WHERE x != array[]::xml[]
> >
> >
> Thanks Scott but that throws up a syntax error (at the closing bracket
> of array[]):
>
>    ERROR:  syntax error at or near "]"
>    LINE 3: AS x FROM docs) AS y WHERE x != array[]::xml[]

Even if that syntax was correct it wouldn't work, xml values don't have
an equality operator defined for them.  I've normally tested the array
size to figure out when they're empty, something like:

  array_upper($1,1) > 0

However, I've just noticed that this returns NULL rather than zero as
I was expecting for an empty array (i.e. the literal '{}'). It also
doesn't seem to do useful things if you're using unusual bounds on your
array.

Bah, the semantics of arrays in PG always seem over-complicated to me!

--
  Sam  http://samason.me.uk/

Sam Mason wrote:
> On Sun, Jul 12, 2009 at 06:41:57PM +0100, Roy Walter wrote:
>> Scott Bailey wrote:
>>> Roy Walter wrote:
>>>> How do I test for an empty array in postgres?
>>> WHERE x != array[]::xml[]
>>>
>>>
>> Thanks Scott but that throws up a syntax error (at the closing bracket
>> of array[]):
>>
>>    ERROR:  syntax error at or near "]"
>>    LINE 3: AS x FROM docs) AS y WHERE x != array[]::xml[]
>
> Even if that syntax was correct it wouldn't work, xml values don't have
> an equality operator defined for them.  I've normally tested the array
> size to figure out when they're empty, something like:
>
>   array_upper($1,1) > 0
>
> However, I've just noticed that this returns NULL rather than zero as
> I was expecting for an empty array (i.e. the literal '{}'). It also
> doesn't seem to do useful things if you're using unusual bounds on your
> array.
>
> Bah, the semantics of arrays in PG always seem over-complicated to me!

This worked on both 8.3 and 8.4

SELECT *
FROM (
   VALUES( '{}'::_xml),('{<root/>}'::_xml)
) sub
WHERE array_upper(column1, 1) > 0Sam Mason <sam@samason.me.uk>


And as Sam noted, array_upper needs to return null if the array is empty
because 0 and -1 can be valid indexes for arrays in postgres.

Sam Mason wrote:

On Sun, Jul 12, 2009 at 06:41:57PM +0100, Roy Walter wrote:

Scott Bailey wrote:

Roy Walter wrote:

How do I test for an empty array in postgres?

WHERE x != array[]::xml[]

 

Thanks Scott but that throws up a syntax error (at the closing bracket of array[]):

   ERROR:  syntax error at or near "]"
   LINE 3: AS x FROM docs) AS y WHERE x != array[]::xml[]

Even if that syntax was correct it wouldn't work, xml values don't have
an equality operator defined for them.  I've normally tested the array
size to figure out when they're empty, something like:

  array_upper($1,1) > 0

However, I've just noticed that this returns NULL rather than zero as
I was expecting for an empty array (i.e. the literal '{}'). It also
doesn't seem to do useful things if you're using unusual bounds on your
array.

Bah, the semantics of arrays in PG always seem over-complicated to me!

This worked on both 8.3 and 8.4

SELECT *
FROM (
  VALUES( '{}'::_xml),('{<root/>}'::_xml)
) sub
WHERE array_upper(column1, 1) > 0Sam Mason <sam@samason.me.uk>


And as Sam noted, array_upper needs to return null if the array is empty because 0 and -1 can be valid indexes for arrays in postgres.

SELECT x
FROM (SELECT xpath('//entry[contains(p, ''searchtext'')]/@*', docxml)
AS x FROM docs) AS y WHERE x <> [test for empty array?????????]

On Sun, Jul 12, 2009 at 09:49:15PM +0100, Roy Walter wrote:
> Where exactly does that fit in terms of my original query, i.e.:
>
>    SELECT x
>    FROM (SELECT xpath('//entry[contains(p, ''searchtext'')]/@*', docxml)
>    AS x FROM docs) AS y WHERE x <> [test for empty array?????????]

Something like the following should do the right thing:

  SELECT x FROM (
    SELECT xpath('//entry[contains(p, ''searchtext'')]/@*', docxml) AS x
    FROM docs) AS y
  WHERE array_upper(x, 1) > 0;

--
  Sam  http://samason.me.uk/

On Sun, Jul 12, 2009 at 09:49:15PM +0100, Roy Walter wrote: 

Where exactly does that fit in terms of my original query, i.e.:
  SELECT x  FROM (SELECT xpath('//entry[contains(p, ''searchtext'')]/@*', docxml)  AS x FROM docs) AS y WHERE x <> [test for empty array?????????]   

Something like the following should do the right thing:
 SELECT x FROM (   SELECT xpath('//entry[contains(p, ''searchtext'')]/@*', docxml) AS x   FROM docs) AS y WHERE array_upper(x, 1) > 0;