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https://issues.apache.org/jira/browse/SOLR-8220?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15031289#comment-15031289
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Keith Laban commented on SOLR-8220:
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bq. From a performance perspective, reading values from DocValues always (if
they exist) can be horrible because each field access in docvalues may need a
random disk seek, whereas, all stored fields for a document are kept together
and need only 1 random seek and a sequential block read.
I agree that reading values from DocValues always can be horrible, my line of
thinking was that if you are asking for one or two fields from a large document
and they are both dv and stored reading from dv would likely be much more
efficient. It might make more sense to be able to get those values explicitly
from docValues using the the transformer, or have some logic that can determine
when it is more efficient. That should be discussed further in [SOLR-8344].
At this point the question that remains; should we move forward with these
patches and move logic for retrieving dv fields to SolrIndexSearcher, leaving
out {{\*}}, {{\*\_foo}} and other _optimizations_ for now? i.e. retrieve fields
by name, if they exist in dv, but are not stored.
> 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, 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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