Performance bottlenecks in a query using SQL
The script needs query rowsource statistics to produce output, so make sure the query is run with STATISTICS_LEVEL = ALL or gather_plan_statistics hint before analyzing it with the script.
Script output:
id, operation — id and description of the plan operation
potential_savings — how much performance benefits can be expected from tuning this operation (e.g. by creating or enabling a better access path)
origin_of_inefficiency — an educated guess as to the origin of inefficiency based on how many times the operation was executed (STARTS), whether or not there are filter predicates for this operation, and some additional information about parent operations
starts — “starts” column from V$SQL_PLAN_STATISTICS
potential_savings — how much performance benefits can be expected from tuning this operation (e.g. by creating or enabling a better access path)
origin_of_inefficiency — an educated guess as to the origin of inefficiency based on how many times the operation was executed (STARTS), whether or not there are filter predicates for this operation, and some additional information about parent operations
starts — “starts” column from V$SQL_PLAN_STATISTICS
rem (c) Nikolay Savvinov, 2013
rem a script to identify performance bottleneck
rem and estimate potential benefits from removing them
rem using rowsource statistics (will only return meaningful results
rem if the plan is still in the cache and if rowsource statistics were populated.
rem takes sql_id and child_number as parameters
set verify off
set linesize 400
set pagesize 9999
column id format 999
column operation format A40
column potential_savings format A30
column origin_of_inefficiency format A30
column comments format A30
column starts format 999999
column filter_predicates format A30
column repetition_ratio format 999999
spool report.txt
with myplan as
(
select p.sql_id, id, parent_id, operation, options, object_owner, object_name, object_type, cardinality, depth,
S.LAST_CR_BUFFER_GETS gets, S.last_starts STARTS, s.last_output_rows outrows, p.filter_predicates, p.access_predicates
from v$sql_plan p,
v$sql_plan_statistics s
where p.sql_id = s.sql_id
and P.CHILD_NUMBER = S.CHILD_NUMBER
and p.id = s.operation_id
and (p.sql_id, p.child_number) = (SELECT * FROM (SELECT SQL_ID, CHILD_NUMBER FROM V$SQL WHERE SQL_TEXT like '&&1' ORDER BY LAST_ACTIVE_TIME DESC) WHERE ROWNUM=1)
-- and p.sql_id = '&1'
-- and p.child_number = &2
),
extended_plan as
(
select mp.*,
(
select max(id)
from myplan
connect by prior id = parent_id
start with id = mp.id
and 'TABLE ACCESS' = mp.operation
and mp.options like 'BY%ROWID'
) rowid_provider_id,
(
select min(id)
from myplan
where parent_id = mp.id
) older_child_id,
(select min(id) from myplan where parent_id = mp.id) older_child,
(select count(*) from myplan where parent_id = mp.id) num_children
from myplan mp
),
opstats as
(
select owner, table_name, num_rows, blocks, t.stale_stats, t.user_stats
from dba_tab_statistics t
where table_name in (select object_name from myplan)
and partition_name is null
union
select owner, index_name, num_rows, leaf_blocks blocks, I.STALE_STATS, I.USER_STATS
from dba_ind_statistics i
where index_name in (select object_name from myplan)
and partition_name is null
),
segstats as
(
select owner, segment_name, sum(blocks) actual_blocks
from dba_segments s
where segment_name in (select object_name from myplan)
and rownum>0
group by owner, segment_name
),
inrows as
(
select lpad(' ', ep.depth, ' ') || operation || ' ' || options || ' ' plan_output,
(select min(id) from myplan where operation like 'PARTITION%' and id != ep.id connect by prior parent_id = id and num_children<=1 start with id = ep.id) partition_iterator_id,
case when operation in ('FILTER', 'INDEX', 'VIEW', 'TABLE ACCESS', 'COUNT') then gets
when operation = 'HASH JOIN' then gets - (select sum(gets) from extended_plan ep2 where parent_id = ep.id)
end own_gets,
case when operation = 'TABLE ACCESS' and options like 'BY%ROWID' then (select outrows from myplan where id = rowid_provider_id)
-- when operation = 'TABLE ACCESS' and options = 'FULL' then ss.num_rows*ep.starts
when operation = 'TABLE ACCESS' and options = 'FULL' then ceil(num_rows*gets/os.blocks)
when (operation = 'INDEX' and options like '%SCAN' and options != 'FULL SCAN') then ceil(num_rows*gets/os.blocks)
else (select outrows from myplan where id = older_child_id)
end inrows,
ep.*,
ceil(gets/os.blocks) rpt_ratio,
os.num_rows,
os.blocks,
ss.actual_blocks,
os.user_stats,
os.stale_stats
from extended_plan ep,
opstats os,
segstats ss
where ep.object_owner = os.owner (+)
and ep.object_name = os.table_name (+)
and ep.object_owner = ss.owner (+)
and ep.object_name = ss.segment_name (+)
),
leakage as
(
select inrows.*,
case when nvl(inrows,0) <= 0 then null
when inrows1 then 'inefficient filtering operation (probably a subquery could not be unnested)'
when operation = 'FILTER' and filter_predicates is not null and num_children=1 then 'restrictive single-source filtering operation'
when operation = 'INDEX' and starts = 1 and filter_predicates is not null then 'index filter predicates'
when operation = 'INDEX' and starts > 1 and partition_iterator_id is null and filter_predicates is null then 'inefficient driving operation (join, subquery etc.) and/or B-tree navigation overhead'
when operation = 'INDEX' and starts > 1 and partition_iterator_id is not null and filter_predicates is null and (select operation || ' ' || options from leakage where id = l.partition_iterator_id) not like '%SINGLE%' then 'navigating multiple partitions of a local index (' || (select operation || ' ' || options from leakage where id = l.partition_iterator_id) || ')'
when operation = 'INDEX' and starts > 1 and partition_iterator_id is not null and filter_predicates is null and (select operation || ' ' || options from leakage where id = l.partition_iterator_id) like '%SINGLE%' then 'inefficient driving operation'
when operation = 'INDEX' and starts > 1 and partition_iterator_id is not null and filter_predicates is null then 'navigating multiple partitions of a local index (' || (select operation || ' ' || options from leakage where id = l.partition_iterator_id) || ')'
when operation = 'INDEX' and starts > 1 and partition_iterator_id is not null and filter_predicates is not null then 'inefficient index access predicates, navigating multiple partitions of a local index (' || (select operation || ' ' || options from leakage where id = l.partition_iterator_id) || ')'
when operation = 'INDEX' and starts > 1 and filter_predicates is not null then 'inefficient join order and/or access predicates'
when operation = 'TABLE ACCESS' and options = 'FULL' and starts = 1 and filter_predicates is not null then 'rows rejected by filter predicate after a full table scan'
when operation = 'TABLE ACCESS' and options = 'FULL' and starts > 1 then 'multiple executions of a full table scan'
when operation = 'TABLE ACCESS' and options like 'BY%ROWID' and starts > 1 and filter_predicates is null then 'inefficient driving operation (join, subquery etc.)'
when operation = 'TABLE ACCESS' and options like 'BY%ROWID' and starts>1 and filter_predicates is not null then 'inefficient driving operation (join, subquery etc.) and/or access predicates'
when operation = 'TABLE ACCESS' and options like 'BY%ROWID' and starts=1 and filter_predicates is not null then 'inefficient access predicates'
WHEN COST_LEAKAGE>0 THEN 'unknown'
end leakage_reason,
round(abs(100*(actual_blocks/blocks - 1))) stats_error,
nullif(rpt_ratio, 1) repetition_ratio,
max(gets) over () total_gets
from leakage l
)
select id,
operation || ' ' || options || ' ' || object_owner || decode(object_name, null, null, '.') || object_name operation, --num_rows, gets, inrows, outrows,
nvl2(cost_leakage, cost_leakage || ' (' || round(100*cost_leakage/gets) || '% operation, ' || round(100*cost_leakage/total_gets) || '% query)', 'UNKNOWN') potential_savings,
-- round(abs(100*(actual_blocks/blocks - 1))) || case when user_stats = 'YES' then ' caution: user-defined stats!' when stale_stats = 'YES' then ' caution: stale stats!' end "stats",
-- stale_stats,
-- user_stats,
case when cost_leakage is null then notes else leakage_reason end origin_of_inefficiency,
case when stats_error>10 then 'caution: actual blocks differ from estimated blocks by ' || stats_error || ' %' end || case when user_stats = 'YES' then '(user-defined stats!)' when stale_stats = 'YES' then '(stale stats!)' end comments,
starts/*,
repetition_ratio,
filter_predicates*/
from summary s
where gets>0
--where cost_leakage > 0
order by cost_leakage desc nulls first;
spool off
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