Mauro Pagano's Blog

Something new about SQL Plan Directives and 12.2


SQL Plan Directives (SPD) remind me of bind peeking about 10 years ago, a nice feature on paper that worked fine most of the times but caused some major headache on those occasions when it didn’t. Luckily for bind peeking social media wasn’t that popular 10y ago so it took a while to “make a name” for a troublemaking feature, nowadays a couple of blog posts and many re-tweets to get pretty popular, poor SPD!
DISCLAIMER: I happen to like a lot both features and think positively of them, I just admit the “transparent” behavior was a little too ambitious 😦

Just like for bind peeking (Adaptive Cursor Sharing), Oracle introduced some changes in 12.2 to mitigate the effect of SQL Plan Directives and Adaptive Dynamic Sampling (DS). Parameter OPTIMIZER_ADAPTIVE_FEATURES has been split in two, basically turning SPD off via parameter OPTIMIZER_ADAPTIVE_STATISTICS,  as Franck Pachot already explained here.

The goal of this post is look into what happens when we manually turn on OPTIMIZER_ADAPTIVE_STATISTICS. Focus will be on SQL Plan Directives alone instead of throwing DBMS_STATS in the mix too (especially since fix for bug 21171382 disables automatic column group creation based on info from SPD).

Let’s create a table with strong correlation across columns and see how 12.2 SPD behaves differently than 12.1

drop table tab1 purge;
create table tab1 (n1 number, n2 number, n3 number);
insert into tab1 select mod(rownum, 100), mod(rownum, 100), mod(rownum, 100) from dual connect by rownum <= 100000;
exec dbms_stats.gather_table_stats(user,'TAB1');

Table TAB1 has 100k rows with three numeric columns, all storing the same exact value and I’ll use the following SQL referencing all the three columns to lead the CBO into a trap (caused by lack of column group on the three columns).

select count(*) from tab1 where n1 = 1 and n2 = 1 and n3 = 1;

Let’s run the SQL a couple times flushing SPD and shared_pool after each execution just so that each change is persisted immediately and we can see the impact.

Run the SQL the first time and from DBA_SQL_PLAN_DIRECTIVES we have:

TYPE                    STATE      REASON                               NOTES 
----------------------- ---------- ------------------------------------ -------------------------------------------------------
                                                                         <spd_text>{EC(MPAGANO.TAB1)[N1, N2, N3]}</spd_text> 

Same result from both 12.1 and 12.2, nothing too outstanding happens behind the curtains either beside the recursive SQLs related to SPD now have a nice “/* QOSD */” comment in there so they are easier to spot.

Run the SQL again, the CBO recognizes it doesn’t have sufficient info to make a good decisions thus ADS is triggered and SPD <internal_state> is updated to MISSING_STATS, pretty much same behavior in both 12.1 and 12.2 except in 12.2 there is no RESULT_CACHE hint in the DS SQL.
This is where things get interesting, in 12.2 there is one additional directive created to store the result of DS:

TYPE                    STATE      REASON                               NOTES
----------------------- ---------- ------------------------------------ ----------------------------------------------------------------------------------------------------
                                                                         <spd_text>{(MPAGANO.TAB1, num_rows=100000) - (SQL_ID:7pjdz422db2xk, T.CARD=1000[-2 -2])}</spd_text>

which allows the result not to be flushed out and survive a restart of the database, thus reducing the amount of DS SQL executed.

The spd_text provides info about the original number of rows (100k) and the number of rows returned by DS (1k), and it includes a SQL ID (7pjdz422db2xk) that sounds like the DS one, even though it’s just a slight variation of it, let’s look into it.
The recursive DS SQL executed is 0nkvqpzha4x88

PARSING IN CURSOR #140370085840592 len=301 dep=1 uid=106 oct=3 lid=106 tim=588259148047 hv=3768743176 ad='73b99738' sqlid='0nkvqpzha4x88'
SELECT /* DS_SVC */ /*+ dynamic_sampling(0) no_sql_tune no_monitoring optimizer_features_enable(default) no_parallel */ NVL(SUM(C1),0) FROM (SELECT /*+ qb_name("innerQuery") NO_INDEX_FFS( "TAB1") */ 1 AS C1 FROM "TAB1" "TAB1" WHERE ("TAB1"."N1"=1) AND ("TAB1"."N2"=1) AND ("TAB1"."N3"=1)) innerQuery

but the fix for 22760704  says “Generate ADS stmt cache key without hints to make it determinist”, which makes it safe to guess the SQL ID stored is a stripped down version of the original SQL text, with the goal of increasing the chance of matching it back when retrieving info from the SPD DS result.

It seems the CBO tries to find DS result using the SQL text of the DS SQL that it would like to execute (SQL ID 0nkvqpzha4x88)

SPD: qosdGetFObjKeyFromStmt: sqlText = SELECT /* DS_SVC */ /*+ dynamic_sampling(0) no_sql_tune no_monitoring optimizer_features_enable(default) no_parallel  */ NVL(SUM(C1),0) FROM (SELECT /*+ qb_name("innerQuery") NO_INDEX_FFS( "TAB1")  */ 1 AS C1 FROM "TAB1" "TAB1" WHERE ("TAB1"."N1"=1) AND ("TAB1"."N2"=1) AND ("TAB1"."N3"=1)) innerQuery (objid = 8846687189427653554)
>> Single Tab Card adjusted from 0.100000 to 1000.000000 due to adaptive dynamic sampling

but internally it modifies the SQL to search for such stripped down version without the hints in the external query block, this is visible via trace[ADS]

kkoadsComputeSqlid: sql_id=8846687189427653554: newText=SELECT /* DS_SVC */  NVL(SUM(C1),0) FROM (SELECT /*+ qb_name("innerQuery") NO_INDEX_FFS( "TAB1")  */ 1 AS C1 FROM "TAB1" "TAB1" WHERE ("TAB1"."N1"=1) AND ("TAB1"."N2"=1) AND ("TAB1"."N3"=1)) innerQuery, startPos = 20, stopPos = 120, newTextLen = 203, sqlLen = 302

and this is the SQL_ID 7pjdz422db2xk stored in the SPD.

It’s too early to know if these changes will address the existing concerns, especially since many sites will run with this feature disabled (by default) but I really like the way the “new” SPD seems to work!!

In summary the most notable changes in 12.2 seems to be:

  • SPD still triggeres ADS but the result is stored inside SPD itself and persisted
  • Search in the SPD repository for DS results are made using a key that represent (a cleaned version of) the DS SQL that generated the result in the first place

As usual, feedbacks / corrections / additions are very much welcome!


2 thoughts on “Something new about SQL Plan Directives and 12.2

  1. Pingback: Latch free waits, Dynamic Statistics, and the Result Cache | DBA n00b

  2. Thanks for doing this investigation! Very interesting. You also tipped me off to the small change where dynamic stats queries are no longer using the result cache hint, so that will no longer be a performance side effect from dynamic stats even if you turn the parameter back on after patching.

    Liked by 1 person

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