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https://issues.apache.org/jira/browse/SOLR-8208?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15270428#comment-15270428
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Cao Manh Dat commented on SOLR-8208:
------------------------------------

+1 The last patch sound great! Turn out that SearchHandler distribute the 
search for us.

> DocTransformer executes sub-queries
> -----------------------------------
>
>                 Key: SOLR-8208
>                 URL: https://issues.apache.org/jira/browse/SOLR-8208
>             Project: Solr
>          Issue Type: Improvement
>            Reporter: Mikhail Khludnev
>            Assignee: Mikhail Khludnev
>              Labels: features, newbie
>         Attachments: SOLR-8208.diff, SOLR-8208.patch, SOLR-8208.patch, 
> SOLR-8208.patch, SOLR-8208.patch, SOLR-8208.patch, SOLR-8208.patch, 
> SOLR-8208.patch, SOLR-8208.patch, SOLR-8208.patch, SOLR-8208.patch, 
> SOLR-8208.patch, SOLR-8208.patch, SOLR-8208.patch, SOLR-8208.patch, 
> SOLR-8208.patch
>
>
> The initial idea was to return "from" side of query time join via 
> doctransformer. I suppose it isn't  query-time join specific, thus let to 
> specify any query and parameters for them, let's call it sub-query. But it 
> might be problematic to escape subquery parameters, including local ones, 
> e.g. what if subquery needs to specify own doctransformer in &fl=\[..\] ?
> I suppose we can specify subquery parameter prefix:
> {code}
> ..&q=name_s:john&fl=*,depts:[subquery fromIndex=departments]&
> depts.q={!term f=dept_id_s 
> v=$row.dept_ss_dv}&depts.fl=text_t,dept_id_s_dv&depts.rows=12&depts.sort=id 
> desc
> {code}       
> response is like
> {code}       
> <response>
> ...
>     <result name="response" numFound="1" start="0">
>         <doc>
>             <str name="id">1</str>
>             <str name="name_s_dv">john</str>
> ..
>             <result name="depts" numFound="2" start="0">
>                 <doc>
>                     <str name="dept_id_s_dv">Engineering</str>
>                     <str name="text_t">These guys develop stuff</str>
>                 </doc>
>                 <doc>
>                     <str name="dept_id_s_dv">Support</str>
>                     <str name="text_t">These guys help users</str>
>                 </doc>
>             </result>
>         </doc>
>     </result>
> </response>
> {code}       
> * {{fl=depts:\[subquery]}} executes a separate request for every query result 
> row, and adds it into a document as a separate result list. The given field 
> name (here it's 'depts') is used as a prefix to shift subquery parameters 
> from main query parameter, eg {{depts.q}} turns to {{q}} for subquery, 
> {{depts.rows}} to {{rows}}.
> * document fields are available as implicit parameters with prefix {{row.}} 
> eg. if result document has a field {{dept_id}} it can be referred as 
> {{v=$row.dept_id}} this combines well with \{!terms} query parser   
> * {{separator=','}} is used when multiple field values are combined in 
> parameter. eg. a document has multivalue field {code}dept_ids={2,3}{code}, 
> thus referring to it via {code}..&dept.q={!terms f=id 
> v=$row.dept_ids}&..{code} executes a subquery {code}{!terms f=id}2,3{code}. 
> When omitted  it's a comma. 
> * {{fromIndex=othercore}} optional param allows to run subquery on other 
> core, like it works on query time join
> However, it doesn't work on cloud setup (and will let you know), but it's 
> proposed to use regular params ({{collection}}, {{shards}} - whatever, with 
> subquery prefix as below ) to issue subquery to a collection
> {code}
> q=name_s:dave&indent=true&fl=*,depts:[subquery]&rows=20&
> depts.q={!terms f=dept_id_s v=$row.dept_ss_dv}&depts.fl=text_t&
> depts.indent=true&
> depts.collection=departments&
> depts.rows=10&depts.logParamsList=q,fl,rows,row.dept_ss_dv
> {code}
> Caveat: it should be a way slow; it handles only search result page, not 
> entire result set. 



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