Hi all,
I have been trying to optimize the performance of LowessNormalization
plugin a little bit lately. It is extremely slow on our Postgres
database when using small 'block groups'.
Anyway, I came across something that surprised me. When using
DynamicResultIterator I noticed that hasNext() does not behave in the
way I expect. After fetching a row by next() method, invoking hasNext()
causes iterator to move to the next row. I checked that on the following
example (it = iterator):
command | result
-----------------------
r = it.next() | -
r.getInt(pos) | 324
it.hasNext() | true
r.getInt(pos) | 325
-----------------------
I looked into the code to check what is going on.
DynamicResultIterator.hasNext() invokes next() method on
java.sql.ResultSet object to check if there are rows to read, but it
does not move the cursor back(ResultSet.previous()). In addition, next()
method uses hasNext() to move the cursor forward (otherwise executing
if(it.hasNext) r=it.next() would skip one row).
Is this how it is meant to be? I realize that the usual way of using
hasNext() is in the context like:
while(it.hasNext)
r=it.next()
// process r
where this issue is not a problem, but sometimes it is useful to check
if there are next rows of data without actually moving the cursor. One
could also benefit from previous() method in that DynamicResultIterator
class.
There are probably reasons behind this kind of implementation I am not
aware of. Then mentioning in the docs that hasNext moves the pointer to
next result would be nice.
Best wishes,
Pawel
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