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From "J.J. Larrea" <>
Subject Merging with IndexWriter.addIndexes(...)
Date Tue, 29 Nov 2005 00:06:57 GMT
My application needs to simultaneously process record additions and 
updates with one pass through a database. That's not in itself a 
problem: I open an IndexReader on the existing index to mark the 
prior versions of updated records as deleted Documents, and an 
IndexWriter on a new empty index to accept the new and updated 
records as new Documents.  Then it's simply a matter of merging the 
two indexes into one.

One possibility is to directly merge from the IndexReader (existing 
index) into the IndexWriter (new index).  The problem is that it 
takes a very long time, on the order of several hours, since it must 
replicate everything in the old index (2.7Gb, 2.2M Documents) in the 
new one.  Since the update consists of a small number of records, 
typically under 1000 and sometimes just a handful, this is a waste of 

Anticipating that, my code reverses the relationship, closing the 
index objects at the end of the loop then opening an IndexWriter on 
the existing (target) index and an IndexReader on the index which has 
the new records, which gets merged into the target.  It functions as 
expected and desired, but is also far more time-consuming than 

For example, if I take an empty (no Document) or small (100 
Documents) index and merge one or other via IndexWriter.addIndexes() 
into an already optimized single-segment index, what should in theory 
be a null or trivial operation ends up in practice unpacking the 
destination index into many segments, and then repacking them into a 
single segment. For that 2.7Gb index it takes over an hour.

In contrast, it only a few seconds to add those same 100 Documents 
directly to the index via addDocument(), as long as one doesn't 

So... I notice that both IndexWriter.addIndexes(...) merge methods 
start and end with calls to optimize() on the target index.  I'm not 
sure whether that is causing the unpacking and repacking I observe, 
but it does wonder whether they truly need to be there:

- Is starting off with zero or 1 segment (per a comment in the source 
code) truly a precondition for successful merging?

- Since one can open an unoptimized index and add millions of 
Documents (creating potentially hundreds of segments) via 
addDocument, with optimization left entirely optional and up to the 
user, why is optimization required as a postcondition for merging?

Any advice on this or the general application design, would be appreciated.

J.J. Larrea

PS: This was tested against SVN trunk revision 329490

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