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 '$prefixed-data' variable removed. Here is the optimized query in BaseX 7.7 beta:
Compiling:
- rewriting fn:exists($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 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