Groovy parallel sql queries - sql

Question

I'm trying to run parallel sql queries using GPars. But somehow it isn't working as I expected. Since I'm relatively new to groovy/java concurrency I'm not sure how to solve my issue.
I following code:
def rows = this.sql.rows(
"SELECT a_id, b_id FROM data_ids LIMIT 10 OFFSET 10"
)
With this code I get a List of ids. Now I want the load the data for each loaded id and this should happen parallel to improve my performance, because I have a large database.
To get the detail data I use the following code:
GParsPool.withPool() {
result = rows.collectParallel {
// 2. Get the data for each source and save it in an array.
def tmpData = [:]
def row = it
sql.withTransaction {
if (row.a_id != null) {
tmpData.a = sql.firstRow("SELECT * FROM data_a WHERE id = '" + row.a_id + "'")
}
if (row.b_id != null) {
tmpData.b = sql.firstRow("SELECT * FROM data_b WHERE id = '" + row.b_id + "'")
}
}
return tmpData
}
// 3. Return the loaded data.
return result
Now I run the code and everything works fine except that the code isn't executed parallel. Using the JProfiler I can see that I have blocked threads and waiting threads, but 0 runnable threads.
Thanks for any help. I you need more information, I will provide them :)
Daniel