Solution for indexing sys_category in ext:solr in TYPO3 with unmerged https://review.typo3.org/#/c/22415/ and unfixed http://forge.typo3.org/issues/44961

gistfile1.txt

plugin.tx_solr.index.queue {
  pages.fields {
  	tags_textM = SOLR_MULTIVALUE
  	tags_textM {
  		cObject = CONTENT
  		cObject {
  			table = pages
  			select {
  				uidInList = this
  				pidInList = 0
  				selectFields = sys_category.*
  				join = sys_category_record_mm ON pages.uid = sys_category_record_mm.uid_foreign JOIN sys_category ON sys_category.uid = sys_category_record_mm.uid_local
  				where = sys_category_record_mm.tablenames = 'pages' AND sys_category.pid = {$uid.pages.tags}
  			}
  			renderObj = COA
  			renderObj {
  				10 = TEXT
  				10.field = title
  				10.wrap = |,
  			}
  		}
  		separator = ,
  		removeEmptyValues = 1
  	}
  }


  news.fields {
    tags_textM = SOLR_MULTIVALUE
    tags_textM {
    	cObject = CONTENT
    	cObject {
    		table = tx_news_domain_model_news
    		select {
    			uidInList.field = uid
    			pidInList = 0
    			selectFields = sys_category.*
    			join = sys_category_record_mm ON tx_news_domain_model_news.uid = sys_category_record_mm.uid_foreign JOIN sys_category ON sys_category.uid = sys_category_record_mm.uid_local
    			where = sys_category_record_mm.tablenames = 'tx_news_domain_model_news' AND sys_category.pid = {$uid.pages.tags}
    		}
    		renderObj = COA
    		renderObj {
    			10 = TEXT
    			10.field = title
    			10.wrap = |,
    		}
    	}
    	separator = ,
    	removeEmptyValues = 1
    }
  }
}