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https://issues.apache.org/jira/browse/SOLR-8220?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14977575#comment-14977575
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Yonik Seeley commented on SOLR-8220:
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bq. reading values from docValues when a stored version of the field does not
exist would be a valuable disk usage optimization.
+1, and I've heard a number of users request this.
bq. 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.
See ReturnFields / ResultContext, that's currently where stored field handling
is centralized, and handles anywhere a field list (or pseudo-fields /
transformers) is specified.
+1 for 2a as a first pass.
For bonus points, prevent stored fields from being loaded at all when not
needed. This gets us a big step closer to having the normal request handler
have the same performance as "/export".
Looking beyond the first pass, it might be nice to use docValues as more of a
first-class alternate "stored" mechanism, and consider them part of "*". If
for some reason it's desirable to treat some docValues fields as stored, and
others not, we could introduce a flag on <field> in the schema.
bq. 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.
This shouldn't be much of a concern for approach 2a, but another future option
would be to add explicit set types, and also implement list-type multi-valued
docValues fields... prob using binary docValues under the covers).
> 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
>
> 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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