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https://issues.apache.org/jira/browse/SOLR-8220?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15012826#comment-15012826
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Yonik Seeley commented on SOLR-8220:
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Back in the day, LazyField actually had a pointer directly into the index where
the field value could be read.
That got remove from Lucene at some point, and was replaced with something just
for compat sake IIRC that had an N^2 bug... doc was loaded on each lazy-field
access, which Hoss found/fixed. But that leaves less performance benefit to
using LazyDocument. On a quick look, it seems to load all lazy fields at once
when the first lazy field is touched. I guess these days it's more of a memory
optimization than a performance one.
Might be worth considering new approaches (we can break back compat in trunk
for 6.0).
Or maybe subclass LazyDocument and do something different for docValues fields.
> Read field from docValues for non stored fields
> -----------------------------------------------
>
> Key: SOLR-8220
> URL: https://issues.apache.org/jira/browse/SOLR-8220
> Project: Solr
> Issue Type: Improvement
> Reporter: Keith Laban
> Attachments: SOLR-8220-ishan.patch, SOLR-8220-ishan.patch,
> SOLR-8220-ishan.patch, SOLR-8220-ishan.patch, SOLR-8220.patch,
> SOLR-8220.patch, SOLR-8220.patch, SOLR-8220.patch
>
>
> Many times a value will be both stored="true" and docValues="true" which
> requires redundant data to be stored on disk. Since reading from docValues is
> both efficient and a common practice (facets, analytics, streaming, etc),
> reading values from docValues when a stored version of the field does not
> exist would be a valuable disk usage optimization.
> The only caveat with this that I can see would be for multiValued fields as
> they would always be returned sorted in the docValues approach. I believe
> this is a fair compromise.
> I've done a rough implementation for this as a field transform, but I think
> it should live closer to where stored fields are loaded in the
> SolrIndexSearcher.
> Two open questions/observations:
> 1) There doesn't seem to be a standard way to read values for docValues,
> facets, analytics, streaming, etc, all seem to be doing their own ways,
> perhaps some of this logic should be centralized.
> 2) What will the API behavior be? (Below is my proposed implementation)
> Parameters for fl:
> - fl="docValueField"
> -- return field from docValue if the field is not stored and in docValues,
> if the field is stored return it from stored fields
> - fl="*"
> -- return only stored fields
> - fl="+"
> -- return stored fields and docValue fields
> 2a - would be easiest implementation and might be sufficient for a first
> pass. 2b - is current behavior
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