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https://issues.apache.org/jira/browse/SOLR-8220?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15072241#comment-15072241
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Shalin Shekhar Mangar commented on SOLR-8220:
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bq. It didn't have to be this way. It could have designed such that fl=* is
only for stored fields and those explicitly setting useDocValuesAsStored=true.
Let me take a step back. The way I think about useDocValuesAsStored is that it
gives a hint to Solr that this field *can* be retrieved from DocValues. But
that doesn't necessarily mean that we will always retrieve it from DocValues.
Later, in SOLR-8344 we will implement some heuristics to choose whether to
retrieve from stored or from DV if both are enabled.
So, by making useDocValuesAsStored=true as default, we don't tie our hands in
SOLR-8344 and we'd be free to choose as we desire. But if the user wants a
field to never be *automatically* retrieved from DocValues then he can set
useDocValuesAsStored=false. Keep in mind that this is only for automatic
selection and if the user wants to explicitly retrieve a field by specifying
its full name (no globbing) in the 'fl' parameter then we respect his/her
wishes and retrieve from DV if necessary.
Maybe useDocValuesAsStored is a bad name and 'autoDocValuesAsStored' conveys
the meaning better?
> 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
> Assignee: Shalin Shekhar Mangar
> Attachments: SOLR-8220-5x.patch, SOLR-8220-branch_5x.patch,
> 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,
> SOLR-8220.patch, SOLR-8220.patch, SOLR-8220.patch, SOLR-8220.patch,
> SOLR-8220.patch, SOLR-8220.patch, SOLR-8220.patch, SOLR-8220.patch,
> SOLR-8220.patch, SOLR-8220.patch, SOLR-8220.patch, SOLR-8220.patch,
> SOLR-8220.patch, SOLR-8220.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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