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https://issues.apache.org/jira/browse/LUCENE-2335?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12847766#action_12847766
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Toke Eskildsen commented on LUCENE-2335:
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The sort-first-then-resolve-Strings is what I did in the proof of concept. The
speed is that of TermsInfoReader, where it delivers a Term from a given
position. If this is too slow for multiple segments, the segment-spanning
ordered ordinals-approach could be tried.
As for deprecating stored fields, then I guess there's the issue of spatial
locality. Wouldn't moving the bytes into the inverted term index bloat it in a
way that makes all searches slower?
There's an issue of having multiple terms in the same field for a given
document, which also ties into facets. It takes some more logic to handle this,
but I think it can be done without excessive memory or processing load:
Basically we make two passes, where the first pass determines the optimal
packed structure and the second pass fills in the ordinals. This would give us
a memory overhead of
{code}
#docs + #references_to_terms + #terms ints
{code}
for very fast facet structure building with support for collator sorted terms
in the facet result. This is basically what we're already doing at
Statsbiblioteket - the only real difference is whether the Strings are pulled
from the Terms index or from an external structure.
Saving RAM, this could be be done using PackedInts
{code}
#docs*log2(#references_to_terms) + #references_to_terms*log2(#terms) +
#terms*log2(#terms) bits
{code}
but I am afraid that access time would suffer. A hybrid
{code}
#docs*32 + #references_to_terms*32 + #terms*log2(#terms) bits
{code}
would be just as fast for building as the non-packed version and a wee bit
slower for the final fetching of the terms.
Of course, just as with fillFields=true searches, the calculated Terms must be
extracted at the end. For faceting, this can be quite a load.
The facet-supporting structure is not as simple as the sorting-optimized one. I
realize that supporting facets from the start might be quite a large jump.
However, if API-breaks are requires, I guess it would be best to do it as few
times as possible?
> optimization: when sorting by field, if index has one segment and field
> values are not needed, do not load String[] into field cache
> ------------------------------------------------------------------------------------------------------------------------------------
>
> Key: LUCENE-2335
> URL: https://issues.apache.org/jira/browse/LUCENE-2335
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Search
> Reporter: Michael McCandless
> Priority: Minor
> Fix For: 3.1
>
>
> Spinoff from java-dev thread "Sorting with little memory: A suggestion",
> started by Toke Eskildsen.
> When sorting by SortField.STRING we currently ask FieldCache for a
> StringIndex on that field.
> This can consumes tons of RAM, when the values are mostly unique (eg a title
> field), as it populates both int[] ords as well as String[] values.
> But, if the index is only one segment, and the search sets fillFields=false,
> we don't need the String[] values, just the int[] ords. If the app needs to
> show the fields it can pull them (for the 1 page) from stored fields.
> This can be a potent optimization -- alot of RAM saved -- for optimized
> indexes.
> When fixing this we must take care to share the int[] ords if some queries do
> fillFields=true and some =false... ie, FieldCache will be called twice and it
> should share the int[] ords across those invocations.
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