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From "Michael McCandless" <luc...@mikemccandless.com>
Subject Re: Flexible indexing design (was Re: Pooling of posting objects in DocumentsWriter)
Date Thu, 10 Apr 2008 09:50:38 GMT
Michael Busch <buschmic@gmail.com> wrote:

> > I agree we would have an abstract base Posting class that just tracks
> > the term text.
> >
> > Then, DocumentsWriter manages inverting each field, maintaining the
> > per-field hash of term Text -> abstract Posting instances, exposing
> > the methods to write bytes into multiple streams for a Posting in the
> > RAM "byte slices", and then read them back when flushing, etc.
> >
> > And then the code that writes the current index format would plug into
> > this and should be fairly small and easy to understand.  For example,
> > frq/prx postings and term vectors writing would be two plugins to the
> > "inverted terms" API; it's just that term vectors flush after every
> > document and frq/prx flush when RAM is full.
> >
> >
>
>  I think this makes sense. We also need to come up with a good solution for
> the dictionary, because a term with frq/prx postings needs to store two (or
> three for skiplist) file pointers in the dictionary, whereas e. g. a
> "binary" posting list only needs one pointer.

Right.  I had been thinking at a minimum we allow "flexibility" by
storing N offsets instead of hardwiring frq and prx offsets alone.  N
is 2 now (frq and prx), but could change eg if we put skip into a
separate file like KS does then N = 3.  If you don't store positions
then N drops back to 2, etc.  This would at least be a start.

> > Then there would also be plugins that just tap into the entire
> > document (don't need inversion), like FieldsWriter.
> >
> > There are still alot of details to work out...
> >
>
>  Definitely. For example, we should think about the Field APIs. Since we
> don't have global field semantics in Lucene I wonder how to handle conflict
> cases, e. g. when a document specifies a different posting list format than
> a previous one for the same field. The easiest way would be to not allow it
> and throw an exception. But this is kind of against Lucene's way of dealing
> with fields currently. But I'm scared of the complicated code to handle
> conflicts of all the possible combinations of posting list formats.
> KinoSearch doesn't have to worry about this, because it has a static schema
> (I think?), but isn't as flexible as Lucene.

Yes, assuming we keep this flexibility, then it's up to each plugin to
"deal" with this 1) when writing docs and 2) when merging segments.

We are going to have to make the FieldInfo API generic, somehow, so
that plugins can record interesting details into the FieldInfo.  EG
the addition of payloads required adding a "storePayloads" boolean
into FieldInfo.  Likewise, in LUCENE-1231 you need to record into
FieldInfo whether the fixed or variable length encoding is in use.

So we need extensibility of FieldInfo too: multiple plugins need to
store stuff.

> > > The DocumentsWriter does pooling of the Posting instances and I'm
> wondering how much this improves performance.
> > >
> >
> > We should retest this.  I think it was a decent difference in
> > performance but I don't remember how much.  I think the pooling can
> > also be made generic (handled by DocumentsWriter).  EG the plugin
> > could expose a "newPosting()"  method.
> >
> >
>
>  Yeah, but for code simplicity let's really figure out first how much
> pooling helps at all.

OK I will test this at some point.

Mike

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