问为什么索引会增加postgres中的执行时间？

EN

select workflow.* from 
    ( 
        SELECT  CASE 
    WHEN cert.value = 'true' THEN 0 
    ELSE 1 END lawAndOrder, workflow.*  
    FROM testdb.statuschanges hist, testdb.usercerts cert, testdb.workflow 
    WHERE workflow.taskid = hist.taskid AND hist.username = cert.username  AND cert.projectname = 'Core_EECA_RUS_2018_shp_UPDGeo_LL'  AND cert.key = 'DISABLECOMMIT' 
    AND hist.statuschangeid =workflow.proposedstatuschangeid AND workflow.status in ('Check')and workflow.projectname = 'Core_EECA_RUS_2018_shp_UPDGeo_LL'  AND workflow.claimedby IS NULL
    UNION 
    SELECT CASE
    WHEN workflow.status = 'INProgress' THEN 1
    WHEN workflow.status = 'Feedback' THEN 2 
    WHEN workflow.status not     in ('Check', 'INProgress', 'Feedback') THEN 3 
    END lawAndOrder,workflow.*  
    FROM testdb.workflow  
    WHERE       workflow.status != 'Check' AND       workflow.claimedby IS NULL  AND workflow.projectname = 'Core_EECA_RUS_2018_shp_UPDGeo_LL'  order by lawAndOrder ASC, TaskId
    ) 
    workflow WHERE ProjectName = 'Core_EECA_RUS_2018_shp_UPDGeo_LL'  AND
    Status IN  ('SourcedFromGuidedCommunity', 'NeedsWorkInTIF', 'NeedsFieldCollection', 'INProgress', 'Completed', 'Check', 'New', 'Reject', 'In Testing', 'Check Reject', 'Check Accept with minor remark', 'InQA', 'Rework', 'Feedback') 
    AND ClaimedBy IS NULL

CREATE INDEX idx_workflow_status
  ON testdb.workflow
  USING btree
  (status COLLATE pg_catalog."default");