xpath() subquery 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[]

Scott Bailey wrote:

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[]

------------------------------------------------------------------------

No virus found in this incoming message.
Checked by AVG - www.avg.com
Version: 8.5.387 / Virus Database: 270.13.12/2233 - Release Date: 07/12/09 08:20:00

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[]
^

-- Roy

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.

Scott Bailey wrote:

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.

Thanks, that's great, I'm delighted that there's a solution. 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?????????]

-- Roy

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/

Sam Mason wrote:

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;

Thank you, works just fine. I thought it was more complicated than that :-)

-- Roy