[Solr Wiki] Update of "DataImportHandlerDeltaQueryViaFullImport" by ShawnHeisey

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The "DataImportHandlerDeltaQueryViaFullImport" page has been changed by ShawnHeisey:
https://wiki.apache.org/solr/DataImportHandlerDeltaQueryViaFullImport?action=diff&rev1=4&rev2=5

Comment:
Clean up some phrasing.  The original sentence about deletedPkQuery is VERY confusing, I did my best at interpreting intent.

  = Using query attribute for both full and delta import =
- The standard approach in Solr is to define one query for the initial import and a second query to fetch the IDs of documents that have changed and a third query to fetch the data that changed. Especially if you expect a large number of changes this isn't very efficient. Furthermore if both in the initial import and the delta case you have the same SELECT list, its tedious to maintain 3 queries where two are almost identical and one still very similar. The fundamental idea is to only define one query for both the full and delta import by reading out the requests parameters on a full-import.
+ The standard approach in Solr is to define one query for the initial import and a second query to fetch the IDs of documents that have changed and a third query to fetch the data that changed. If you expect a large number of changes this isn't very efficient. Furthermore if both in the initial import and the delta case you have the same SELECT list, its tedious to maintain 3 queries where two are almost identical and one still very similar. The fundamental idea is to only define one query for both the full and delta import, using SQL syntax to make the queries different based on the {{{clean}}} parameter.
  
  == Example ==
  So take the example from the docs:
@@ -22, +22 @@

      WHERE '${dataimporter.request.clean}' != 'false'
        OR last_modified > '${dataimporter.last_index_time}'">
  }}}
- When doing a normal full import solr defaults the {{{clean}}} to {{{true}}} (watch out you can override this default in the solrconfig.xml so you might want to always set it explicitly to true). As a result the first part of the WHERE condition will be {{{'true' != 'false'}}} which any decent RDBMS will figure out will lead to the entire all always evaluating to true aka reading the entire table: {{{http://localhost:8983/solr/core0/dataimport?command=full-import&clean=true}}}
+ When doing a normal full import solr defaults the {{{clean}}} to {{{true}}}.   You can make this default explicit in the solrconfig.xml handler definition, in the "defaults" section. As a result the first part of the WHERE condition will be {{{'true' != 'false'}}}, which is always true.  Most database servers will use that to evaluate the entire WHERE condition to true, ignoring the second clause after the "OR", so all rows will match.  {{{http://localhost:8983/solr/core0/dataimport?command=full-import&clean=true}}}
  
- Now when doing a delta import you do not use the delta import, but instead you do a normal full-import but with the {{{clean}}} GET parameter set to {{{false}}}: {{{http://localhost:8983/solr/core0/dataimport?command=full-import&clean=false}}}
+ Now when doing a delta import you do not use the delta-import command, but instead you do a normal full-import but with the {{{clean}}} url parameter set to {{{false}}}: {{{http://localhost:8983/solr/core0/dataimport?command=full-import&clean=false}}}  In this case the first part of the WHERE will be {{{'false' != 'false'}}} which is always false.  The "OR" clause is then evaluated to decide whether the row matches.
  
+ When want to use {{{deletedPkQuery}}}, running the {{{delta-import}}} command is still necessary, which means that you will still need {{{deltaQuery}}} and {{{deltaImportQuery}}}.
- In this case the first part of the WHERE will be {{{'false' != 'false'}}} which is obviously always false and any RDBMS should optimize that away and just evaluate the second condition.
- 
- In this case it means obviously that in case you also want to use {{{deletedPkQuery}}} then when running the {{{delta-import}}} command is still necessary.
  
  == Efficiency Aspect ==
  There might be situations where separate queries might be more efficient. Consider an example where the query that fetches the document data is a very complex join and the RDBMS doesn't do a great job with coming up with a good query plan and you can determine a relatively small number of ID's that change every night with a SELECT without a JOIN, then it might be more efficient to do one query to fetch the ID's and then one query per fetched ID that does the complex join.