{"id":6617,"date":"2015-12-27T20:42:01","date_gmt":"2015-12-27T19:42:01","guid":{"rendered":"https:\/\/www.dbi-services.com\/blog\/phantom-session-environment-in-multitenant\/"},"modified":"2015-12-27T20:42:01","modified_gmt":"2015-12-27T19:42:01","slug":"phantom-session-environment-in-multitenant","status":"publish","type":"post","link":"https:\/\/www.dbi-services.com\/blog\/phantom-session-environment-in-multitenant\/","title":{"rendered":"Phantom session environment in multitenant?"},"content":{"rendered":"

By Franck Pachot<\/h2>\n

.
\nIn multitenant a session can switch from one container to another. There are a few restrictions such as a transaction cannot span multiple containers. But how do sessions parameters behave? It seems that there is a small inconsistency.
\n
\nI was looking at some adaptive plan and SPM behavior (see this tweeter conversation<\/a>) and played with the optimizer environment to reproduce the plan I wanted, and then came to a strange behavior. I can understand that a session keep its parameter settings when switching to another container. But not when closing and re-opening the pluggable database. The default values should come then.<\/p>\n

Here is an example.<\/p>\n

Change in memory value of optimizer parameter<\/h3>\n

This is in 12.1.0.2 latest PSU. I connect to PDB<\/p>\n

\n19:13:10 SQL> alter session set container=PDB;\nSession altered.\n<\/code><\/pre>\n
and check parameter optimizer_index_cost_adj from memory:<\/p>\n
\n19:13:10 SQL> show parameter optimizer_index_cost_adj;\n \nNAME                                 TYPE        VALUE\n------------------------------------ ----------- ------------------------------\noptimizer_index_cost_adj             integer     100\n<\/code><\/pre>\n
It’s the default.<\/p>\n
then I set it to one in memory only:<\/p>\n
\n19:37:17 SQL> alter system set optimizer_index_cost_adj=1 scope=memory;\nSystem altered.\n<\/code><\/pre>\n
And I run a query and check that the optimizer used that value<\/p>\n
\n19:37:18 SQL> select * from dual;\n \nD\n-\nX\n \n19:37:19 SQL> select * from table(dbms_xplan.display_cursor(format=>'+outline'));\n \nPLAN_TABLE_OUTPUT\n--------------------------------------------------------------------------\nSQL_ID  a5ks9fhw2v9s1, child number 0\n-------------------------------------\nselect * from dual\n \nPlan hash value: 272002086\n \n--------------------------------------------------------------------------\n| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |\n--------------------------------------------------------------------------\n|   0 | SELECT STATEMENT  |      |       |       |     2 (100)|          |\n|   1 |  TABLE ACCESS FULL| DUAL |     1 |     2 |     2   (0)| 00:00:01 |\n--------------------------------------------------------------------------\n \nOutline Data\n-------------\n \n  \/*+\n      BEGIN_OUTLINE_DATA\n      IGNORE_OPTIM_EMBEDDED_HINTS\n      OPTIMIZER_FEATURES_ENABLE('12.1.0.2')\n      DB_VERSION('12.1.0.2')\n      OPT_PARAM('optimizer_index_cost_adj' 1)\n      ALL_ROWS\n      OUTLINE_LEAF(@\"SEL$1\")\n      FULL(@\"SEL$1\" \"DUAL\"@\"SEL$1\")\n      END_OUTLINE_DATA\n  *\/\n \n<\/code><\/pre>\n
Si this is expected, my custom value has been used to optimize the cursor.<\/p>\n
close and re-open the PDB<\/h3>\n
I close and restart my PDB:<\/p>\n
\n19:37:19 SQL> shutdown immediate\nPluggable Database closed.\n19:37:21 SQL> startup\nPluggable Database opened.\n<\/code><\/pre>\n
As the value was set in memory, it’s now back to default:<\/p>\n
\n19:36:48 SQL> show parameter optimizer_index_cost_adj;\n \nNAME                                 TYPE        VALUE\n------------------------------------ ----------- ------------------------------\noptimizer_index_cost_adj             integer     100\n \n<\/code><\/pre>\n
and of course, my cursor is not there anymore:<\/p>\n
\n19:36:48 SQL> select sql_id,child_number,first_load_time,con_id from v$sql where sql_id='a5ks9fhw2v9s1';\n \nno rows selected\n<\/code><\/pre>\n
So now I run again my query and expect the default optimizer environement:<\/p>\n
\n19:36:48 SQL> select dummy from dual;\n \nD\n-\nX\n \n19:36:48 SQL> select * from table(dbms_xplan.display_cursor(format=>'+outline'));\n \nPLAN_TABLE_OUTPUT\n---------------------------------------------------------------------------\nSQL_ID  4au7rzs3y6kzn, child number 0\n-------------------------------------\nselect dummy from dual\n \nPlan hash value: 272002086\n \n--------------------------------------------------------------------------\n| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |\n--------------------------------------------------------------------------\n|   0 | SELECT STATEMENT  |      |       |       |     2 (100)|          |\n|   1 |  TABLE ACCESS FULL| DUAL |     1 |     2 |     2   (0)| 00:00:01 |\n--------------------------------------------------------------------------\n \nOutline Data\n-------------\n \n  \/*+\n      BEGIN_OUTLINE_DATA\n      IGNORE_OPTIM_EMBEDDED_HINTS\n      OPTIMIZER_FEATURES_ENABLE('12.1.0.2')\n      DB_VERSION('12.1.0.2')\n      OPT_PARAM('optimizer_index_cost_adj' 1)\n      ALL_ROWS\n      OUTLINE_LEAF(@\"SEL$1\")\n      FULL(@\"SEL$1\" \"DUAL\"@\"SEL$1\")\n      END_OUTLINE_DATA\n  *\/\n<\/code><\/pre>\n
And my new cursor has used the optimizer_index_cost_adj=1 which is not what I expected at all.
\nDoes that mean that session parameters remain across PDB close\/start?
\nNo according to ‘show parameter’:<\/p>\n
\n19:37:07 SQL> show parameter optimizer_index_cost_adj;\n \nNAME                                 TYPE        VALUE\n------------------------------------ ----------- ------------------------------\noptimizer_index_cost_adj             integer     100\n \n<\/code><\/pre>\n
but yes according to V$SES_OPTIMIZER_ENV:<\/p>\n
\n19:37:07 SQL> select sid,name,value from v$ses_optimizer_env where name='optimizer_index_cost_adj' and sid=sys_context('userenv','sid');\n \n       SID NAME                                     VALUE\n---------- ---------------------------------------- -------------------------\n        35 optimizer_index_cost_adj                 1\n \n<\/code><\/pre>\n
Conclusion<\/h3>\n
I’ve reproduced the same when re-connecting (instead of changing session container) and even when connecting to another PDB.
\nIn my opinion, this is a bug. The session parameters should be reinitialized when re-connecting. So be careful.<\/p>\n
Update 12-JUL-2016<\/h3>\n
See Maciej Tokar post<\/a> about this issue (my case being only one effect)<\/p>\n","protected":false},"excerpt":{"rendered":"
By Franck Pachot . In multitenant a session can switch from one container to another. There are a few restrictions such as a transaction cannot span multiple containers. But how do sessions parameters behave? It seems that there is a small inconsistency.<\/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":[220,64,96,209,66,223],"type_dbi":[],"class_list":["post-6617","post","type-post","status-publish","format-standard","hentry","category-database-administration-monitoring","tag-cdb","tag-multitenant","tag-oracle","tag-oracle-12c","tag-pdb","tag-pluggable-databases"],"acf":[],"yoast_head":"\nPhantom session environment in multitenant? - 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\/phantom-session-environment-in-multitenant\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Phantom session environment in multitenant?\" \/>\n<meta property=\"og:description\" content=\"By Franck Pachot . 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