{"id":9645,"date":"2017-01-05T15:52:46","date_gmt":"2017-01-05T14:52:46","guid":{"rendered":"https:\/\/www.dbi-services.com\/blog\/12cr2-no-cardinality-feedback-for-small-queries\/"},"modified":"2017-01-05T15:52:46","modified_gmt":"2017-01-05T14:52:46","slug":"12cr2-no-cardinality-feedback-for-small-queries","status":"publish","type":"post","link":"https:\/\/www.dbi-services.com\/blog\/12cr2-no-cardinality-feedback-for-small-queries\/","title":{"rendered":"12cR2: no cardinality feedback for small queries"},"content":{"rendered":"<h2>By Franck Pachot<\/h2>\n<p>.<br \/>\nThis follows the comments on Clemens Bleile post about SQL Plan Directives in Oracle 12.2 where he discovered a new behavior that is not documented. The feature is good: bypass the cardinality feedback overhead when the query is fast anyway.<br \/>\n<!--more--><\/p>\n<h3>Feature<\/h3>\n<p>In Clemens Bleile post about <a href=\"http:\/\/dbi-services.com\/blog\/sql-plan-directives-in-12cr2-behavior-is-different-than-in-12cr1\/\">SQL Plan Directives changes in 12<em>c<\/em>R2<\/a> he determined than the new feature coming in bug 23596611 was responsible for the bypass of automatic reoptimization (the 12<em>c<\/em> cardinality feedback) in some cases.<\/p>\n<p>The description gives the idea that the reason for the bypass of cardinality feedback is &#8216;small queries&#8217;:<\/p>\n<pre><code>SQL&gt; select BUGNO,SQL_FEATURE,DESCRIPTION,OPTIMIZER_FEATURE_ENABLE from v$system_fix_control where bugno=23596611;\n&nbsp;\n     BUGNO SQL_FEATURE                    DESCRIPTION                            OPTIMIZER_FEATURE_ENABLE\n---------- ------------------------------ -------------------------------------- -------------------------\n  23596611 QKSFM_STATS_FEEDBACK_23596611   No feedback for small queries         12.2.0.1<\/code><\/pre>\n<p>This is a new concept and in the traces we know there is nothing about &#8216;small queries&#8217;. Here is how to get more information.<\/p>\n<h3>Function<\/h3>\n<p>First, I&#8217;m looking for the &#8216;small queries&#8217; text in the oracle executable:<\/p>\n<pre><code>$ strings $ORACLE_HOME\/bin\/oracle | grep \"small query\"\nkkocfbCheckCardEst: No feedback for small query (BG %f, CPU %f (ms)\nkkocfbCompareExecStats: skipping cost comparision forsmall query (BG %f, CPU %f (ms)<\/code><\/pre>\n<p>Good. This looks like there is a way to trace some information about that bypass decision, and there&#8217;s a clue that &#8216;small queries&#8217; are evaluated on number of buffer gets (BG) and CPU time threshold. More than that, we have the name of the C function that traces this: kkocfbCheckCardEst<\/p>\n<p>kkocfb probably means:<\/p>\n<blockquote class=\"twitter-tweet\" data-width=\"500\" data-dnt=\"true\">\n<p lang=\"en\" dir=\"ltr\"><a href=\"https:\/\/twitter.com\/johnnyq72?ref_src=twsrc%5Etfw\">@johnnyq72<\/a> it\u2019s kernel kompile costing cardinality feedback probably. and it seems there is a trace function&#8230;<\/p>\n<p>&mdash; Frits Hoogland (@fritshoogland) <a href=\"https:\/\/twitter.com\/fritshoogland\/status\/816763076800380929?ref_src=twsrc%5Etfw\">January 4, 2017<\/a><\/p><\/blockquote>\n<p><script async src=\"https:\/\/platform.twitter.com\/widgets.js\" charset=\"utf-8\"><\/script><\/p>\n<p>and then I&#8217;m looking for a way to get this trace. I didn&#8217;t know which event can trace that, but others know:<\/p>\n<blockquote class=\"twitter-tweet\" data-width=\"500\" data-dnt=\"true\">\n<p lang=\"en\" dir=\"ltr\">are you talking about event 10507 level 512?<\/p>\n<p>&mdash; Mauro Pagano (@Mautro) <a href=\"https:\/\/twitter.com\/Mautro\/status\/816772241635627008?ref_src=twsrc%5Etfw\">January 4, 2017<\/a><\/p><\/blockquote>\n<p><script async src=\"https:\/\/platform.twitter.com\/widgets.js\" charset=\"utf-8\"><\/script><\/p>\n<p>However there is another way to get an idea of the events that are checked by a function.<br \/>\nThe idea is described here: <a href=\"http:\/\/yong321.freeshell.org\/computer\/OracleFuncCalls.html\">http:\/\/yong321.freeshell.org\/computer\/OracleFuncCalls.html<\/a><\/p>\n<h3>Event<\/h3>\n<p>We are not allowed to disassemble oracle code (this is in the licensing rules that you accept when you download oracle) so I&#8217;ll not do it here.<br \/>\nIf you think you have a special permission to do it, just run:<\/p>\n<pre><code>gdb oracle &lt;&lt;&lt;&quot;disas kkocfbCheckCardEst&quot;<\/code><\/pre>\n<p>I can&#8217;t show the dissassembled code, so here is how to get the list of the events that are checked by the kkocfbCheckCardEst function:<\/p>\n<pre><code>$ gdb oracle &lt;&lt;&lt;&quot;disas kkocfbCheckCardEst&quot; | awk --non-decimal-data &#039;\/mov .*,%edi$\/{gsub(\/[$,]\/,&quot; &quot;);a=$4}\/EventRdbmsErr\/{printf &quot;%dn&quot;, a}&#039; | sort -u\n10507<\/code><\/pre>\n<p>The information we have about this event does not describe all what can be traced:<\/p>\n<pre><code>$ oerr ora 10507\n10507, 00000, \"Trace bind equivalence logic\"\n\/\/ *Cause:\n\/\/ *Action:<\/code><\/pre>\n<p>From that, I don&#8217;t know which level to set the event, but in case of doubt and because levels are often bit flags, you can set it to the maximum 0xFFFFFFFF:<\/p>\n<pre><code>\nalter session set events '10507 trace name context forever, level 4294967295';\n<\/code><\/pre>\n<h3>trace when cardinality feedback is bypassed<\/h3>\n<p>When I run the query where execution is doing less than 100 buffer gets I get the following trace with event 10507 level 512:<\/p>\n<pre><code>\n****** Begin CFB Hints (APA) [sql_id=a71qw8t17qpqq] xsc=0x7fca901f0e30 ctx=0x9415d4f8 ******\nDumping Hints\n=============\n*********** End CFB Hints (APA) ***********\nkkocfbInitCardFdbkCompCtx [sql_id=a71qw8t17qpqq] monitor=y\nkkocfbCheckCardEst: No feedback for small query (BG 0.000000, CPU 0.000000 (ms)\n<\/code><\/pre>\n<p>The message is clear: no feedback for small query. The Buffer Get and CPU show 0 but on my test case, increasing the size of the table, I have seen that 100 buffer gets is the threshold:<\/p>\n<pre><code>\nbq4fc1rdx97av is_reoptimizable: N                buffer_gets:          95 cpu_time:          0\nbq4fc1rdx97av is_reoptimizable: N                buffer_gets:          95 cpu_time:       1000\nbq4fc1rdx97av is_reoptimizable: N                buffer_gets:          96 cpu_time:       1000\nbq4fc1rdx97av is_reoptimizable: N                buffer_gets:          96 cpu_time:       1000\nbq4fc1rdx97av is_reoptimizable: N                buffer_gets:          97 cpu_time:          0\nbq4fc1rdx97av is_reoptimizable: N                buffer_gets:          97 cpu_time:          0\nbq4fc1rdx97av is_reoptimizable: N                buffer_gets:          98 cpu_time:       1000\nbq4fc1rdx97av is_reoptimizable: N                buffer_gets:          98 cpu_time:          0\nbq4fc1rdx97av is_reoptimizable: N                buffer_gets:          99 cpu_time:       1000\nbq4fc1rdx97av is_reoptimizable: N                buffer_gets:          99 cpu_time:          0\nbq4fc1rdx97av is_reoptimizable: Y                buffer_gets:         100 cpu_time:       1000\nbq4fc1rdx97av is_reoptimizable: Y                buffer_gets:         100 cpu_time:       1000\nbq4fc1rdx97av is_reoptimizable: Y                buffer_gets:         101 cpu_time:       1000\nbq4fc1rdx97av is_reoptimizable: Y                buffer_gets:         101 cpu_time:          0\nbq4fc1rdx97av is_reoptimizable: Y                buffer_gets:         102 cpu_time:          0\nbq4fc1rdx97av is_reoptimizable: Y                buffer_gets:         102 cpu_time:       1000\n<\/code><\/pre>\n<h3>trace when query becomes reoptimizable<\/h3>\n<p>When the query reads more than 100 buffer gets, the cursor is marked as reoptimizable and here is the trace:<\/p>\n<pre><code>\n****** Begin CFB Hints (APA) [sql_id=a71qw8t17qpqq] xsc=0x7fed7dc2ca40 ctx=0x9418f4f8 ******\nDumping Hints\n=============\n*********** End CFB Hints (APA) ***********\nkkocfbInitCardFdbkCompCtx [sql_id=a71qw8t17qpqq] monitor=y\nReparsing due to card est...\n @=0x95426550 type=1 nodeid=1 monitor=Y halias=\"DEMO_TABLE\" loc=\"SEL$1\" act=500 min=0 est=63 next=(nil)\nkkocfbCheckCardEst [sql_id=a71qw8t17qpqq] reparse=y ecs=n efb=n ost=n fbs=n\n *********** Begin Dump Context (kkocfbCheckCardEst) [sql_id=a71qw8t17qpqq cpcnt=0] ***********\n @=0x95426550 type=1 nodeid=1 monitor=Y halias=\"DEMO_TABLE\" loc=\"SEL$1\" act=500 min=0 est=63 next=(nil)\n *********** End Dump Context ***********\n<\/code><\/pre>\n<p>act=500 is the actual number of rows (A-Rows) and est=63 is the estimated one (A-Rows) and this is how misestimate is detected.<\/p>\n<h3>trace when reoptimized query is run again<\/h3>\n<p>At that point we can see the feedback as OPT_ESTIMATE hints in v$sql_reoptimization_hints.<\/p>\n<p>When we run the same query another time, it is parsed again with those hits, fixing cardinality tho the A-Rows of previous execution.<\/p>\n<p>Here is the trace for this second execution.<\/p>\n<pre><code>\nkkoarCopyCtx: [sql_id=a71qw8t17qpqq] origin=CFB old=0x63904140 new=0x7fcda3716d40 copyCnt=1 copyClient=y\n**************************************************************\nkkocfbCopyBestEst: Best Stats\n  Exec count:         1\n  CR gets:            279\n  CU gets:            4\n  Disk Reads:         0\n  Disk Writes:        0\n  IO Read Requests:   0\n  IO Write Requests:  0\n  Bytes Read:         0\n  Bytes Written:      0\n  Bytes Exchanged with Storage:  0\n  Bytes Exchanged with Disk:  0\n  Bytes Simulated Read:  0\n  Bytes Simulated Returned:  0\n  Elapsed Time: 6998 (us)\n  CPU Time: 2000 (us)\n  User I\/O Time: 0 (us)\n *********** Begin Dump Context (kkocfbCopyBestEst) **********\n *********** End Dump Context ***********\nkkocfbCopyCardCtx: oldCtx Dumping string mapping\n----------------------\nkkocfbCopyCardCtx: newCtx Dumping string mapping\n----------------------\n**************************************************************\nkkocfbCopyCardCtx: Best stats\n  Exec count:         1\n  CR gets:            279\n  CU gets:            4\n  Disk Reads:         0\n  Disk Writes:        0\n  IO Read Requests:   0\n  IO Write Requests:  0\n  Bytes Read:         0\n  Bytes Written:      0\n  Bytes Exchanged with Storage:  0\n  Bytes Exchanged with Disk:  0\n  Bytes Simulated Read:  0\n  Bytes Simulated Returned:  0\n  Elapsed Time: 6998 (us)\n  CPU Time: 2000 (us)\n  User I\/O Time: 0 (us)\n **** Begin Dump Context (kkocfbCopyCardCtx - best est) ****\n ********************* End Dump Context ********************\n   *********** Begin Dump Context (Copy:old) [sql_id=a71qw8t17qpqq cpcnt=0] ***********\n   @=0x6446dc58 type=1 nodeid=1 monitor=Y halias=\"DEMO_TABLE\" loc=\"SEL$1\" act=500 min=0 est=63 next=(nil)\n   *********** End Dump Context ***********\n   *********** Begin Dump Context (Copy:new) [sql_id=a71qw8t17qpqq cpcnt=0] ***********\n   @=0x7fcda3716a78 type=1 nodeid=1 monitor=Y halias=\"DEMO_TABLE\" loc=\"SEL$1\" act=500 min=0 est=63 next=(nil)\n   *********** End Dump Context ***********\nkkoarReparse: xsc=0x7fcda3672b98 kxsAutoReoptCtx=0x7fcda3716d40\nkkocfbAddCardHints: Dumping string mapping\n----------------------\n******** Begin CFB Hints [sql_id=a71qw8t17qpqq]  xsc=0x7fcda3672b98 ********\nDumping Hints\n=============\n  atom_hint=(@=0x7fcda37831e8 err=0 resol=0 used=0 token=1018 org=6 lvl=3 txt=OPT_ESTIMATE (@\"SEL$1\" TABLE \"DEMO_TABLE\"@\"SEL$1\" ROWS=500.000000 ) )\n********** End CFB Hints **********\n<\/code><\/pre>\n<p>You can see the OPT_ESTIMATE hints here.<\/p>\n<pre><code>\n****** Begin CFB Hints (APA) [sql_id=a71qw8t17qpqq] xsc=0x7fcda3672b98 ctx=0x8a274118 ******\nDumping Hints\n=============\n  atom_hint=(@=0x7fcda37831e8 err=0 resol=1 used=0 token=1018 org=6 lvl=3 txt=OPT_ESTIMATE (@\"SEL$1\" TABLE \"DEMO_TABLE\"@\"SEL$1\" ROWS=500.000000 ) )\n  atom_hint=(@=0x7fcda3782d10 err=0 resol=1 used=0 token=1018 org=6 lvl=3 txt=OPT_ESTIMATE (@\"SEL$1\" TABLE \"DEMO_TABLE\"@\"SEL$1\" ROWS=500.000000 ) )\n*********** End CFB Hints (APA) ***********\nkkocfbInitCardFdbkCompCtx [sql_id=a71qw8t17qpqq] monitor=y\n**************************************************************\nkkocfbCopyBestEst: Best Stats\n  Exec count:         1\n  CR gets:            279\n  CU gets:            4\n  Disk Reads:         0\n  Disk Writes:        0\n  IO Read Requests:   0\n  IO Write Requests:  0\n  Bytes Read:         0\n  Bytes Written:      0\n  Bytes Exchanged with Storage:  0\n  Bytes Exchanged with Disk:  0\n  Bytes Simulated Read:  0\n  Bytes Simulated Returned:  0\n  Elapsed Time: 6998 (us)\n  CPU Time: 2000 (us)\n  User I\/O Time: 0 (us)\n *********** Begin Dump Context (kkocfbCopyBestEst) **********\n *********** End Dump Context ***********\nkkocfbCheckCardEst [sql_id=a71qw8t17qpqq] reparse=n ecs=n efb=y ost=n fbs=n\n<\/code><\/pre>\n<h3>So what?<\/h3>\n<p>With those adaptive feature it is good to be able to trace the decisions in order to understand and reproduce the problems we encounter. Event 10507 is very useful. It&#8217;s the execution time counterpart for the event 10053 which explains compile time decision. And in latest versions, the optimizer is more and more present at execution time.<\/p>\n<p>We have always seen problems coming from cardinality feedback. Most of them are coming from bad statistics or a data model where there is not one optimal access plan. Then the CBO is always trying to find better and sometimes the best is the enemy of the good.<br \/>\nNow in 12.2 you have a very good way to avoid the cardinality feedback side effect: tune your queries and your data model so that the critical queries read less than 100 logical reads.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>By Franck Pachot . This follows the comments on Clemens Bleile post about SQL Plan Directives in Oracle 12.2 where he discovered a new behavior that is not documented. The feature is good: bypass the cardinality feedback overhead when the query is fast anyway.<\/p>\n","protected":false},"author":27,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[229],"tags":[349,988,1013],"type_dbi":[],"class_list":["post-9645","post","type-post","status-publish","format-standard","hentry","category-database-administration-monitoring","tag-optimizer","tag-oracle-12-2","tag-spd"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO Premium plugin v27.2 (Yoast SEO v27.2) - https:\/\/yoast.com\/product\/yoast-seo-premium-wordpress\/ -->\n<title>12cR2: no cardinality feedback for small queries - dbi Blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.dbi-services.com\/blog\/12cr2-no-cardinality-feedback-for-small-queries\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"12cR2: no cardinality feedback for small queries\" \/>\n<meta property=\"og:description\" content=\"By Franck Pachot . 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The feature is good: bypass the cardinality feedback overhead when the query is fast anyway.\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.dbi-services.com\/blog\/12cr2-no-cardinality-feedback-for-small-queries\/\" \/>\n<meta property=\"og:site_name\" content=\"dbi Blog\" \/>\n<meta property=\"article:published_time\" content=\"2017-01-05T14:52:46+00:00\" \/>\n<meta name=\"author\" content=\"Oracle Team\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Oracle Team\" \/>\n\t<meta name=\"twitter:label2\" content=\"Est. reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"8 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\/\/schema.org\",\"@graph\":[{\"@type\":\"Article\",\"@id\":\"https:\/\/www.dbi-services.com\/blog\/12cr2-no-cardinality-feedback-for-small-queries\/#article\",\"isPartOf\":{\"@id\":\"https:\/\/www.dbi-services.com\/blog\/12cr2-no-cardinality-feedback-for-small-queries\/\"},\"author\":{\"name\":\"Oracle Team\",\"@id\":\"https:\/\/www.dbi-services.com\/blog\/#\/schema\/person\/66ab87129f2d357f09971bc7936a77ee\"},\"headline\":\"12cR2: no cardinality feedback for small queries\",\"datePublished\":\"2017-01-05T14:52:46+00:00\",\"mainEntityOfPage\":{\"@id\":\"https:\/\/www.dbi-services.com\/blog\/12cr2-no-cardinality-feedback-for-small-queries\/\"},\"wordCount\":681,\"commentCount\":0,\"keywords\":[\"Optimizer\",\"Oracle 12.2\",\"SPD\"],\"articleSection\":[\"Database Administration &amp; Monitoring\"],\"inLanguage\":\"en-US\",\"potentialAction\":[{\"@type\":\"CommentAction\",\"name\":\"Comment\",\"target\":[\"https:\/\/www.dbi-services.com\/blog\/12cr2-no-cardinality-feedback-for-small-queries\/#respond\"]}]},{\"@type\":\"WebPage\",\"@id\":\"https:\/\/www.dbi-services.com\/blog\/12cr2-no-cardinality-feedback-for-small-queries\/\",\"url\":\"https:\/\/www.dbi-services.com\/blog\/12cr2-no-cardinality-feedback-for-small-queries\/\",\"name\":\"12cR2: no cardinality feedback for small queries - dbi Blog\",\"isPartOf\":{\"@id\":\"https:\/\/www.dbi-services.com\/blog\/#website\"},\"datePublished\":\"2017-01-05T14:52:46+00:00\",\"author\":{\"@id\":\"https:\/\/www.dbi-services.com\/blog\/#\/schema\/person\/66ab87129f2d357f09971bc7936a77ee\"},\"breadcrumb\":{\"@id\":\"https:\/\/www.dbi-services.com\/blog\/12cr2-no-cardinality-feedback-for-small-queries\/#breadcrumb\"},\"inLanguage\":\"en-US\",\"potentialAction\":[{\"@type\":\"ReadAction\",\"target\":[\"https:\/\/www.dbi-services.com\/blog\/12cr2-no-cardinality-feedback-for-small-queries\/\"]}]},{\"@type\":\"BreadcrumbList\",\"@id\":\"https:\/\/www.dbi-services.com\/blog\/12cr2-no-cardinality-feedback-for-small-queries\/#breadcrumb\",\"itemListElement\":[{\"@type\":\"ListItem\",\"position\":1,\"name\":\"Accueil\",\"item\":\"https:\/\/www.dbi-services.com\/blog\/\"},{\"@type\":\"ListItem\",\"position\":2,\"name\":\"12cR2: no cardinality feedback for small queries\"}]},{\"@type\":\"WebSite\",\"@id\":\"https:\/\/www.dbi-services.com\/blog\/#website\",\"url\":\"https:\/\/www.dbi-services.com\/blog\/\",\"name\":\"dbi Blog\",\"description\":\"\",\"potentialAction\":[{\"@type\":\"SearchAction\",\"target\":{\"@type\":\"EntryPoint\",\"urlTemplate\":\"https:\/\/www.dbi-services.com\/blog\/?s={search_term_string}\"},\"query-input\":{\"@type\":\"PropertyValueSpecification\",\"valueRequired\":true,\"valueName\":\"search_term_string\"}}],\"inLanguage\":\"en-US\"},{\"@type\":\"Person\",\"@id\":\"https:\/\/www.dbi-services.com\/blog\/#\/schema\/person\/66ab87129f2d357f09971bc7936a77ee\",\"name\":\"Oracle Team\",\"image\":{\"@type\":\"ImageObject\",\"inLanguage\":\"en-US\",\"@id\":\"https:\/\/secure.gravatar.com\/avatar\/f711f7cd2c9b09bf2627133755b569fb5be0694810cfd33033bdd095fedba86d?s=96&d=mm&r=g\",\"url\":\"https:\/\/secure.gravatar.com\/avatar\/f711f7cd2c9b09bf2627133755b569fb5be0694810cfd33033bdd095fedba86d?s=96&d=mm&r=g\",\"contentUrl\":\"https:\/\/secure.gravatar.com\/avatar\/f711f7cd2c9b09bf2627133755b569fb5be0694810cfd33033bdd095fedba86d?s=96&d=mm&r=g\",\"caption\":\"Oracle Team\"},\"url\":\"https:\/\/www.dbi-services.com\/blog\/author\/oracle-team\/\"}]}<\/script>\n<!-- \/ Yoast SEO Premium plugin. -->","yoast_head_json":{"title":"12cR2: no cardinality feedback for small queries - dbi Blog","robots":{"index":"index","follow":"follow","max-snippet":"max-snippet:-1","max-image-preview":"max-image-preview:large","max-video-preview":"max-video-preview:-1"},"canonical":"https:\/\/www.dbi-services.com\/blog\/12cr2-no-cardinality-feedback-for-small-queries\/","og_locale":"en_US","og_type":"article","og_title":"12cR2: no cardinality feedback for small queries","og_description":"By Franck Pachot . This follows the comments on Clemens Bleile post about SQL Plan Directives in Oracle 12.2 where he discovered a new behavior that is not documented. 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