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From Paul Elschot <>
Subject Re: Lucene Seaches VS. Relational database Queries
Date Fri, 14 Apr 2006 09:22:01 GMT

A join like operation between Lucene indexes can be done with
(at least) reasonable performance by using a few standard
methods from RDB's: sort before going to disk, and cache
whenever possible. The steps are:
- query the first Lucene index with the low level search API to get the
  Lucene doc nrs. (using HitCollector or TermDocs).
- retrieve the key field values for the second index from the first
  in doc number order. This step will perform better when
  there is as less data stored in the first index. This is normally
  the most performance critical step. (IndexReader.document(n))
- Sort these key values and use them, again with low level API
  to get the doc numbers for the second Lucene index.
  (using TermDocs).
- Build a Filter for the second index from these doc numbers,
  this step usually implies some sorting of document numbers,
  for example by collecting them in a BitSet.
- Use this filter for a text search in the second index.

On Friday 14 April 2006 00:58, Ananth T. Sarathy wrote:
> Erick,
> Don't get me wrong. I agree with you 100 percent on everything you just
> said, and have been advocating what you are saying. I turned to the forum to
> get other peoples thoughts on the issue, feeling that my perspective may be
> a little warped, and wanted to see what the community thinks. I think there
> is a performance issue with or DB that I have never experienced in any other
> project I have worked on, which needs someone with more specific domain
> knowledge to fix.  I think Lucene is fantastic for what we are already using
> it for (searching contents of HTML, colliding the values of database rows to
> make them free text searchable). We have been using it for over 2 years, and
> with very good results (once we got a hang of it).
>   I for one think that native language searches are fundamentally different
> than Discrete Database queries, I am just having a problem trying to explain
> this to some of the people on my team, and wanted to see if there wer eother
> POV out there.

The first step above can start from the results of an RDB query.
Usually, the last text search step is more interactive (fundamentally
different?) than earlier steps, so a filter is used to cache the join result.
If the join needs to be changed slightly it is also quite effective to
cache the retrieved key values from the first index and the retrieved
fields from the second  index.
For the last step (and earlier ones), when two successive searches
retrieve a somewhat overlapping document set, one might also want to
avoid using  the Hits class, because it only caches results for a single
Instead, some LRU caches for retrieved documents and for filters can
be quite effective. The caches can have the index version in their keys to
keep things in sync.

Enough RAM should be available so that the indexes can be accessed
without alternating between them.
Also the disk head should not do anything else when it is using
the sorted inputs to minimize the total seek time.
When the filters start taking too much RAM, have a look here:

Paul Elschot

> Ananth
> On 4/13/06, Erick Erickson <> wrote:
> >
> > On 4/13/06, Ananth T. Sarathy <> wrote:
> > >
> > > No we do have drop downs selects that would allow for the substitution,
> > > but
> > > we also have a free text fields to allow the user to search. That
> > solution
> > > would I think work for the DB query replacement, but you would need a
> > > regular non underscored field to allow for free text.
> > >
> > >
> > Well, as I say, you've solved that problem already. Somewhere, somehow,
> > you
> > have to decide what to do with the "free text" data. Somewhere, somehow,
> > you've got to decide whether "stunt director trainee" means "stunt
> > director"
> > + trainee, stunt + "director trainee", or stunt + director + trainee. Or
> > else you can't form your SQL in the first place. And the query doesn't
> > produce reasonable results if you *do* form the query.
> >
> > If you can form your SQL with distinct "Title = 'blah'" clauses, you can
> > substitute underscores for spaces in the terms. If you can do that, you
> > can
> > ask Lucene to find the terms you indexed with underscores. And if you
> > can't
> > form your SQL queries in the first place, the question is irrelevant.
> >
> > All that said, perhaps a better question is "why is your SQL slow?".
> > Relational databases are really good at this sort of thing. Many smart
> > people have put many, many developer years into making relational
> > databases
> > deal with joins efficiently. Assuming you have the proper indexes etc.
> >
> > As much as I've been impressed with Lucene, I have to ask whether it's
> > relevant to your problem. I have no clue what database you're using, how
> > it's set up, or whether the examples you've given are simplified enough
> > that
> > I don't understand what the *real* problem is. But if your issue isn't
> > really dealing with a full text search, your relational DB should be able
> > to
> > handle it, given the proper wherewithal. Have you done "explain plan" or
> > its
> > equivalent in your DB? Have you tried adding indexes to avoid full table
> > scans? In short, have you fully convinced yourself that your RDB can't
> > handle the problem?
> >
> > I'm *extremely* leery of introducing another "moving part" into a product
> > without fully exhausting the current parts. It's *never* a good thing to
> > add
> > a new step into the process unless you can convince yourself that it
> > solves
> > more problems than it introduces. You've already alluded to keeping the DB
> > and the Lucene indexes in synch. I *guarantee* that there will be other
> > issues that rise up and bite you. *Count* on whatever you think you'll
> > spend
> > in introducing Lucene into your mix (say effort X) costing you *at least*
> > 2X
> > more time/energy than you think. I'd actually give it a multiplier closer
> > to
> > 4X.
> >
> > This is NOT a slam on lucene. But developers often miss the bigger
> > picture.
> > What processes are you going to put in place to keep the Lucene part of
> > the
> > product up to date? How much is it going to cost your company to
> > troubleshoot the Lucene portion? How many company resources are going to
> > be
> > spent answering customer complaints? What is the ongoing maintenance
> > requirement?
> >
> > I like Lucene. I've just persuaded my company to use it in our next
> > product.
> > I've been incredibly impressed with it's architeture and implementation.
> > But
> > it's a text search engine, and shouldn't be confused with a RDB.
> > *Assuming*
> > that the RDB is an integral part of your product, I'd spend a lot of time
> > making that do what I needed before I'd introduce another moving part.
> >
> > All for what it's worth, from an old "C" programmer <G>..
> >
> > Best
> > Erick
> >
> >
> --
> Ananth T Sarathy

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