The Query Optimizer in Oracle Database 19C What's New?

The Query Optimizer in Oracle Database 19c What’s New?

Christian Antognini

@ChrisAntognini antognini.ch Christian Antognini • Senior principal consultant and partner at Trivadis • Focus: get the most out of database engines • Logical and physical database design • Query optimizer • Application performance management • Author of Troubleshooting Oracle Performance

@ChrisAntognini antognini.ch

Agenda • All Environments • Reporting on Hint Usage • Execution Plan Comparison Refer to the Oracle • Online Statistics Gathering Enhancements Database SQL Tuning Guide • Enterprise Edition with Management Packs for technical information • Statistics Maintenance Enhancements about the new features • Real-Time SQL Monitoring for Developers • Enterprise Edition on Exadata The Database Licensing • Automatic Resolution of Plan Regressions Information User Manual • High-Frequency SPM Evolve Advisor Task provides information about • Real-Time Statistics their availability • High-Frequency Automatic Statistics Collection • Quarantine for Runaway SQL Statements • Automatic Indexing Disclaimer • This presentation covers the versions 19.[3-5] • This presentation does not cover the query optimizer features that are specific to the Autonomous Database services All Environments Reporting on Hint Usage • It is an issue to verify whether hints added to SQL statements are actually used • To make it easier, there is a new section in the output generated by the DBMS_XPLAN.DISPLAY* functions • The new section (Hint Report) reports hints into two categories • Valid (used) • Invalid (either unused, unresolved or having syntax error) • For invalid hints, additional information explaining the issue might be provided Execution Plan Comparison • When dealing with the performance of SQL statements, it is rather usual to compare execution plans • The new function DBMS_XPLAN.COMPARE_PLANS implements a logical (i.e. not line by line) comparison of two or more execution plans • The execution plans can be stored in either a plan table, the cursor cache, the AWR, a STS, a SQL baseline, a SQL profile or an advisor task • The report can be generated in either text, HTML or XML format • It does not work in Standard Edition (bug 30488154) • An ORA-13631 is raised Online Statistics Gathering Enhancements • Up to and including 18c, bulk load operations in empty segments do not gather index statistics • As of 19c, according to the documentation, this restriction no longer exists • In practice, because of a bug, it is only partially the case • Statistics on non-unique indexes are not gathered Enterprise Edition with Management Packs Statistics Maintenance Enhancements • Up to and including 18c, ALTER TABLE operations that move, merge and coalesce partitions do not maintain global and partition-level statistics • As of 19c, according to the documentation, this restriction no longer exists • Histograms are not maintained • In practice, because of bugs, it is only partially the case • Most global statistics are not (correctly) maintained • Partition-level statistics are maintained

PART Real-Time SQL Monitoring for Developers • All database users, without specific privileges, can generate real-time monitoring reports about SQL statements they executed • The API to generate the reports is the regular one

SELECT dbms_sql_monitor.report_sql_monitor(sql_id => '...', type => 'active') FROM dual

• There are new (undocumented) views to see which statements were monitored and how they were executed • V$ALL_SQL_MONITOR • V$ALL_SQL_PLAN_MONITOR • V$ALL_SQL_PLAN • V$ALL_ACTIVE_SESSION_HISTORY TUN Enterprise Edition on Exadata On-premises and Exadata Cloud Service Automatic Resolution of Plan Regressions • Up to and including 18c, by default, the SPM Evolve Advisor only considers SQL statements already loaded in SQL plan baselines • As of 19c, the advisor can also consider other SQL statements • Only resource-intensive SQL statements with several execution plans • New SQL plan baselines can be automatically created • To control the feature, use DBMS_SPM.SET_EVOLVE_TASK_PARAMETER • To enable it set ALTERNATE_PLAN_BASELINE to AUTO (default in 19.3) • To disable it set ALTERNATE_PLAN_BASELINE to EXISTING (default in 19.4+) • 19c also improves the selection of the execution plans stored into CC/AWR/STS High-Frequency SPM Evolve Advisor Task • By default, the SPM Evolve Advisor runs daily in the maintenance window • In 19c it can be configured to also run outside of the maintenance window • To control it, the DBMS_SPM.CONFIGURE procedure supports a new parameter • AUTO_SPM_EVOLVE_TASK (OFF, ON, AUTO) • In 19c AUTO is equivalent to ON • When enabled, it runs every hour for no longer than 30 minutes Real-Time Statistics • They do not replace object statistics, their purpose is to augment them between two gatherings • They are limited to specific object statistics and to INSERT/UPDATE/MERGE • Number of rows and blocks at the table level • Low/high value at the column level • They are gathered and used by default • The NO_GATHER_OPTIMIZER_STATISTICS hint prevents their gathering • They are visible in *_TAB_STATISTICS and *_TAB_COL_STATISTICS • NOTES = STATS_ON_CONVENTIONAL_DML • Execution plans based on them are marked (V$SQL_PLAN.OTHER_XML) High-Frequency Automatic Statistics Collection

• By default, the automatic statistics collection task runs daily in the maintenance window • In 19c it can be configured to also run outside of the maintenance window • To control it, the DBMS_STATS.SET_GLOBAL_PREFS procedure supports three new parameter • AUTO_TASK_STATUS (OFF, ON) • AUTO_TASK_INTERVAL (in seconds, default 900, minimum 60) • AUTO_TASK_MAX_RUN_TIME (in seconds, max/default 3600) • DBA_AUTO_STAT_EXECUTIONS provides information about the runs Quarantine for Runaway SQL Statements • The Resource Manager can be instructed to terminate SQL statements that either run for too long or consume excessive resources • When terminating a SQL statement, 19c automatically creates a quarantine configuration that prevents the execution plan to run • Instead, an ORA-56955 (quarantined plan used) is raised • Quarantine configurations • Can be managed and manually created with DBMS_SQLQ • Are visible in DBA_SQL_QUARANTINE • To be used do not necessarily require the activation of a Resource Manager plan • Cursors using them are marked (V$SQL.SQL_QUARANTINE) and externalize the number of avoided executions (V$SQL.AVOIDED_EXECUTIONS) Automatic Indexing ASTS • Brand new, game changer feature Identify candidate indexes • Expert system that automatically manages performance-related Create invisible indexes Drop unused indexes • Requires no external inputs indexes • Unsupervised • 24x7 SPA Make indexes Validate • Automatically adapts to changes visible/unusable performance • For identification and validation purposes, it uses an automatically Create SQL plan baselines managed SQL tuning set for regressions Automatic Indexing – Handling • By default, it is disabled • It can be enabled either in reporting mode or “fully”

dbms_auto_index.configure('AUTO_INDEX_MODE', 'REPORT ONLY')

dbms_auto_index.configure('AUTO_INDEX_MODE', 'IMPLEMENT')

• Few parameters exist • Schema inclusion/exclusion lists • Retention of unused indexes and logs • Storage (tablespace, space budget, compression) • When enabled, it runs every 15 minutes • DBA_AUTO_INDEX_EXECUTIONS • DBMS_AUTO_INDEX.REPORT_(LAST_)ACTIVITY Summary • Licensing limitations make many (interesting) features not generally available • Small, but useful new features or enhancements • Finally DBMS_XPLAN reports hint usage • Online statistics gathering (index statistics + partition maintenance operation) • Real-time SQL monitoring available to everyone • Quarantine for runaway SQL statements • New concepts; real-world experience is needed to critically judge them • Automatic indexing could be a game changer • Real-time statistics • Automatic resolution of plan regressions • Execution plan comparison • Minor enhancements • High-frequency automatic statistics collection and SPM Evolve Advisor task