Shell script to purge Rundeck execution history (extended from @unicolet's script https://gist.github.com/unicolet/af648a97163ce6b44645)

rd-clean.sh

#!/bin/sh

# keep last few executions for each job
KEEP=100

# db connection params
HOST=YOURDBHOST
PORT=YOURDBPORT
USERNAME=YOURDBUSERNAME
PASSWORD=YOURDBPASSWORD
DB=YOURDB

cd /var/lib/rundeck/logs/rundeck

JOBS=`find . -maxdepth 3 -path "*/job/*" -type d`

for j in $JOBS ; do
        echo "Processing job $j"
        ids=`find $j -iname "*.rdlog" | sed -e "s/.*\/\([0-9]*\)\.rdlog/\1/" | sort -n -r`
        declare -a JOBIDS=($ids)

        if [ ${#JOBIDS[@]} -gt $KEEP ]; then
          for job in ${JOBIDS[@]:$KEEP};do
             echo " * Deleting job: $job"
             echo "   rm -rf $j/logs/$job.*"
             rm -rf $j/logs/$job.*
             workflowid=`mysql -h $HOST -P $PORT -u$USERNAME -p$PASSWORD -s -N -e "select workflow_id from execution where id=$job" $DB`
             workflowstepids=`mysql -h $HOST -P $PORT -u$USERNAME -p$PASSWORD -s -N -e "select workflow_step_id from workflow_workflow_step where workflow_commands_id=$workflowid" $DB`
             declare -a WSIDS=($workflowstepids)
             for workflowstepid in $WSIDS ; do
               echo "   mysql -h $HOST -P $PORT -u$USERNAME -p$PASSWORD -e 'delete from workflow_workflow_step where workflow_step_id=$workflowstepid' $DB"
               mysql -h $HOST -P $PORT -u$USERNAME -p$PASSWORD -e "delete from workflow_workflow_step where workflow_step_id=$workflowstepid" $DB
               echo "   mysql -h $HOST -P $PORT -u$USERNAME -p$PASSWORD -e 'delete from workflow_step where id=$workflowstepid' $DB"
               mysql -h $HOST -P $PORT -u$USERNAME -p$PASSWORD -e "delete from workflow_step where id=$workflowstepid" $DB
             done
             echo "   mysql -h $HOST -P $PORT -u$USERNAME -p$PASSWORD -e 'delete from execution where id=$job' $DB"
             mysql -h $HOST -P $PORT -u$USERNAME -p$PASSWORD -e "delete from execution where id=$job" $DB
             echo "   mysql -h $HOST -P $PORT -u$USERNAME -p$PASSWORD -e 'delete from base_report where jc_exec_id=$job' $DB"
             mysql -h $HOST -P $PORT -u$USERNAME -p$PASSWORD -e "delete from base_report where jc_exec_id=$job" $DB
             echo "   mysql -h $HOST -P $PORT -u$USERNAME -p$PASSWORD -e 'delete from workflow where id=$workflowid' $DB"
             mysql -h $HOST -P $PORT -u$USERNAME -p$PASSWORD -e "delete from workflow where id=$workflowid" $DB
          done
        fi
done

In addition to deleting log files, executions and base_reports, this script deletes workflow and workflow steps generated by an execution.
I have also moved db connection params as variables to avoid repetition.
Finally in my case I needed it to use mysql instead of psql.