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From Mark Miller <>
Subject Re: java.lang.OutOfMemoryError: Java heap space when sorting the fields
Date Wed, 19 Mar 2008 20:22:27 GMT
Heres what happens: in order to sort all of the hits you get back on a 
field, you need to get the value of that field for comparisons right? 
Well it turns out that reading a field value from the index is pretty 
slow (its on the disk after all) Lucene will read all of the terms 
in the field off disk once (on the first sort/search request) and cache 
them. So if your field is an Int your talking numDocs*32 bits for your 
cache. For a Long field its numDocs*64. For a String field Lucene caches 
a String array with every unique term and then an int array indexing 
into the term array.

You might think, if I only ask for the top 10 docs, don't i only read 10 
field values? But of course you don't know what docs will be returned as 
each search comes you have to cache them all.
Like the other answer said, one field is going to be roughly 50 MB. You 
can do the math yourself...I ripped the 40 doing some rough stuff in my 
head, but around 50mb is prob closer.

So anyway, you *are* only sorting on the hits you get back...but youll 
get different hits all the time so you still have to cache all the field 
values. The FieldCache class does this, and thats whats taking up the 
RAM. You should have 50 to give it though I would much RAM 
are you giving the JVM?

- mark

sandyg wrote:
> Hi,
> And is it not passibe to sort on the result we get instead of on all the
> values like
> Hits       hits =;
> and it will be good if got sorting on the hits i.e on the result
> because my sorting is based on specific  field and that field should be
> sorted when i click on it.
> thanks for the reply
> markrmiller wrote:
>> To sort on 13mil docs will take like at least 400 mb for the field
>> cache. Thats if you only sort on one can grow fast if you
>> allow multi field sorting.
>> How much RAM are you giving your app?
>> sandyg wrote:
>>> this is my search content
>>> QueryParser parser = new QueryParser("keyword",new StandardAnalyzer());
>>> Query query = parser.parse("1");
>>> Sort sort = new Sort(new SortField(sortField));
>>>            Hits       hits =,sort);
>>> And i had huge data about 13 millions of records
>>> i am not sure y its giving outof memory exception and
>>> no exception when no sorting is done
>>> plz some one help me yar
>>> and also if to increase heap space how to increase it programatically i
>>> had
>>> command prompt
>>> java -Xms<initial heap size>   -Xmx<maximum heap size>
>>> please.....
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