We have been using BaseX 7.7 beta for a while and recently we switched to 7.8.2 and found two queries failed with the existing unit tests. After research, we found that the query is now getting compiled into a different optimized query (from 7.7) and results in evaluating a variable, even if that variable will never be used, as it is inside an IF condition for particular input. Here is the query:

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

declare function local:addPrefix($arg as element()?){

  concat("prefix-", $arg)

};

let $a := 

  <a>

    <b>1</b>

    <b>2</b>

    <b>3</b>

  </a>

let $prefixed-data:= <ele>{local:addPrefix($a/b)}</ele>

return

  if (exists($a/c)) then ( 

    $prefixed-data

  ) else (

    'no input data'

  )

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

In BaseX 7.7beta the output is 'no input data' which is correct.

Here $prefixed-data does not get evaluated in BaseX 7.7 beta, as exists($a/c) is FALSE. The optimized query in GUI also shows '$prefixed-data' variable removed. Here is the optimized query in BaseX 7.7 beta:

Compiling:

- rewriting for tail calls: Q{http://www.w3.org/2005/xquery-local-functions}addPrefix

- rewriting fn:exists($a/c)

- inlining let $prefixed-data := element ele { (local:addPrefix($a/b)) }

- removing variable $prefixed-data

Optimized Query: 

declare function local:addPrefix($arg as element()?) { fn:concat("prefix-", $arg) }; 

let $a := element a { (element b { ("1") }, element b { ("2") }, element b { ("3") }) } return if($a/c) then element ele { (local:addPrefix($a/b)) } else "no input data"

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

In BaseX 7.8.2 the query throws an error.

In 7.8.2, $prefixed-data gets evaluated even though the 'IF" condition is FALSE. See optimized query from GUI here:

Error:

Stopped at C:/source/perforce/PSODepot/health-analyzer/ha-rel504/libs/catalog/src/main/resources/analysis/vsphere/6.5/CO-001/file3, 1/18:

[XPTY0004] Cannot treat item() sequence as element()?: (<b>...</b>, <b>...</b>, <b>...</b>).

Compiling:

- inlining Q{http://www.w3.org/2005/xquery-local-functions}addPrefix#1

- inlining let $arg_3 as element()? := $a_1/b

- simplifying flwor expression

- rewriting fn:exists($a_1/c)

Query:

declare function local:addPrefix($arg as element()?){ concat("prefix-", $arg) }; let $a := <a> <b>1</b> <b>2</b> <b>3</b> </a> let $prefixed-data:= <ele>{local:addPrefix($a/b)}</ele> return if (exists($a/c)) then ( $prefixed-data ) else ( 'no input data' )

Optimized Query:

let $a_1 := element a { (element b { ("1") }, element b { ("2") }, element b { ("3") }) } let $prefixed-data_2 := element ele { (fn:concat("prefix-", ((: element()?, true :) $a_1/b))) } return if($a_1/c) then $prefixed-data_2 else "no input data"

Query plan:

<QueryPlan>

  <GFLWOR>

    <Let>

      <Var name="$a" id="1"/>

      <CElem>

        <QNm value="a" type="xs:QName"/>

        <CElem>

          <QNm value="b" type="xs:QName"/>

          <Str value="1" type="xs:string"/>

        </CElem>

        <CElem>

          <QNm value="b" type="xs:QName"/>

          <Str value="2" type="xs:string"/>

        </CElem>

        <CElem>

          <QNm value="b" type="xs:QName"/>

          <Str value="3" type="xs:string"/>

        </CElem>

      </CElem>

    </Let>

    <Let>

      <Var name="$prefixed-data" id="2"/>

      <CElem>

        <QNm value="ele" type="xs:QName"/>

        <FNStr name="concat(atom1,atom2[,...])">

          <Str value="prefix-" type="xs:string"/>

          <TypeCheck type="element()?" function="true">

            <IterPath>

              <VarRef>

                <Var name="$a" id="1"/>

              </VarRef>

              <IterStep axis="child" test="b"/>

            </IterPath>

          </TypeCheck>

        </FNStr>

      </CElem>

    </Let>

    <If>

      <IterPath>

        <VarRef>

          <Var name="$a" id="1"/>

        </VarRef>

        <IterStep axis="child" test="c"/>

      </IterPath>

      <VarRef>

        <Var name="$prefixed-data" id="2"/>

      </VarRef>

      <Str value="no input data" type="xs:string"/>

    </If>

  </GFLWOR>

</QueryPlan>

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

We wanted to make sure that this is due to the way queries are executed differently now in BaseX 7.8.2 and not a bug in BaseX 7.8.2. Removing the unused variable makes sense for optimization purposes, so it looks like 7.7 was doing the right thing. Can you confirm please?

Thanks,

Srini