Joins

Relational databases are designed to optimize joins, so DataDirect XQuery leverages the database when an XQuery join involves SQL data. Performing all the joins in the database results in a dramatic performance gain, because DataDirect XQuery simply uses the SQL engine in your database.

Example 4. Join Pushdown

XQuery

for $u in collection('USERS')/USERS
for $h in collection('HOLDINGS')/HOLDINGS
where $u/USERID = $h/USERID
return <holding name="{$u/LASTNAME}">{$h/SHARES/text()}</holding>

Generated SQL

SELECT ALL
 nrm5."LASTNAME" AS RACOL1,
 nrm9."SHARES" AS RACOL2
FROM
 "PEPPINO"."USERS" nrm5,
 "PEPPINO"."HOLDINGS" nrm9
WHERE
 nrm5."USERID" = nrm9."USERID" AND
 LENGTH(nrm5."USERID") = LENGTH(nrm9."USERID")
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