title: Solr atomic/in-place update by CSV
body: <p>I have a core whose columns come from different sources in CSV format (all having a unique ID)</p>
  <p>Source 1:</p>
  <pre><code>ID,Col1,Col2
  1,33,77
  </code></pre>
  <p>Source 2:</p>
  <pre><code>ID,Col3,Col4
  1,8,9
  </code></pre>
  <p>Source 3:</p>
  <pre><code>ID,Col5,Col6
  1,0,100
  </code></pre>
  <p>Uploading CSV will overwrite the entire row including missing columns.</p>
  <pre><code>url 'http://localhost:8983/solr/Data/update?commit=true&amp;escape=\&amp;encapsulator=&quot;&amp;fieldnames=ID,Col1,Col2' -T source1.csv -X POST -H 'Content-type:application/csv'
  url 'http://localhost:8983/solr/Data/update?commit=true&amp;escape=\&amp;encapsulator=&quot;&amp;fieldnames=ID,Col3,Col4' -T source2.csv -X POST -H 'Content-type:application/csv'
  </code></pre>
  <p>Is there a way to use Atomic or In-Place update to add/update specific columns?</p>
  <p>I cannot merge CSVs and I cannot create different cores and merge to one final core.</p>
  <p>I use Solr 8.9.0 on Linux.</p>