using adaptive cursor sharing (acs) to produce multiple optimal plans
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Using Adaptive Cursor Sharing (ACS) to produce multiple Optimal Plans. Carlos Sierra Consulting Technical Advisor. Carlos Sierra. Oracle Server Technologies(ST) Center of Expertise( CoE ). SQL Tuner handyman : developer, advisor, trainer, support - PowerPoint PPT PresentationTRANSCRIPT
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Using Adaptive Cursor Sharing (ACS) to produce multiple Optimal PlansCarlos SierraConsulting Technical Advisor
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Carlos Sierra
SQL Tuner handyman: developer, advisor, trainer, support IT: Oracle(17), UNISYS(12), Ford(3), others(3) Florida(17), Venezuela(3), Puerto Rico(6), Michigan(1), Mexico(X) Tools: SQLTXPLAIN(SQLT), SQLHC, TRCANLZR(TRCA), others Motto: Life is good!
Oracle Server Technologies(ST) Center of Expertise(CoE)
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Adaptive Cursor Sharing (ACS)
Motivation Mechanics Test Case Demo Remarks
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ACS Motivation
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SQL Processing
Hard parse side effects– CPU consumption– Latch contention
Excessive hard parsing – Affects concurrency– Restricts scalability
Mitigating hard parsing– Cursor sharing
Hard parsing is expensive!
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Implementing Cursor SharingReplacing literals with bind variables
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Before Bind Peeking
Predicate– WHERE channel_id = :b1
Unknowns– Is :b1 between low and high values of channel_id?– Is :b1 a popular value of channel_id?– Are there any rows with value :b1 for channel_id?
Penalty– Possible suboptimal plans
Before 9i CBO was blind to values passed
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With Bind Peeking
Predicate– WHERE channel_id = :b1
Plan is determined by peeked values– EXEC :b1 := 9;
Optimal plan for 1st execution– CBO can use low/high and histograms on channel_id
Penalty– Possible suboptimal plans for subsequent executions on skewed data
9i offers a partial solution
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With Adaptive Cursor Sharing
Some queries are ACS candidates Sophisticated non-persistent mechanism Selectivity of predicates determine plan Multiple optimal plans for a query!
– If ACS is successfully applied Penalty
– Marginal increase in CPU and memory overhead
11g improves cursor sharing
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ACS Mechanics
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ACS high-level OverviewHigh level overview
If SQL with binds meets some requirements – Flag cursor as bind sensitive– Start monitoring data volume manipulated by cursor
If bind sensitive and data volume manipulated by cursor varies significantly
– Flag cursor as bind aware– Start generating multiple optimal plans for this query on next hard parse
If bind aware then use selectivity of predicates to decide on plan
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Bind Sensitive
SQL has explicit binds – Or literals and cursor_sharing is “force”
Predicate: column + operand + bind_variable– Equality operand “=“ and histogram on column
Ex: channel_id = :b1– Non-equality operand (range) regardless of histogram on column
“>”, “>=“, “<“, ‘<=“, BETWEEN, LIKE
Minimum requirements
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Bind Aware
Significant changes in data volume manipulated by cursor– A few rows versus a few thousands of rows– A few thousands of rows versus a few millions of rows
Specifying /*+ BIND_AWARE */ CBO Hint– Bypasses the monitoring phase on data volume
How to become bind aware?
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Plan Selection
Evaluate selectivity of predicates at soft parse Compare to a non-persistent selectivity profile If within ranges of a known profile then select associated plan Else hard parse
– Compute and execute newly generated plan– Create selectivity profile for new plan or update profile of existing plan
If ranges on selectivity profiles overlap then merge profiles
Based on selectivity profile of predicates
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V$ dynamic views for ACS
V$SQL– Shareable, bind sensitive and bind aware flags
V$SQL_CS_STATISTICS– Data volume manipulated (rows processed)
V$SQL_CS_HISTOGRAM– Record keeping of data volume per execution (small, medium, large)
V$SQL_CS_SELECTIVITY– Predicates selectivity profiles
ACS non-persistent performance views
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ACS Test Case
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Our Query with LiteralsGuesstimate execution plan then verify it with demo 0
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Possible Access Paths?
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Optimal Execution Plan
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Our Query with Bind VariablesHow many optimal execution plans can you foresee?
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Multiple Optimal Plans for one QueryGuesstimate optimal plan (access paths) for each query
Query :b1 :b2 AP1 AP2q1 9 33 N1 N2
q2 5 32
q3 2 999
q4 9 999
q5 2 33
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Multiple Optimal Plans for one QueryGuesstimate optimal plan (access paths) for each query
Query :b1 :b2 AP1 AP2q1 9 33 N1 N2
q2 5 32 N1 N2
q3 2 999 FTS FTS
q4 9 999 N1 FTS
q5 2 33 FTS N2
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Multiple Optimal Plans for one QueryExecute demos 1-5 and verify access paths
Query :b1 :b2 AP1 AP2q1 9 33 N1 N2q2 5 32 N1 N2q3 2 999 FTS FTSq4 9 999 N1 FTSq5 2 33 FTS N2
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Bind Sensitive: Rows Processed
Data volume manipulated– Fuzzy representation– S: few rows– M: thousands or rows– L: millions of rows
v$sql_cs_histogram– Bucket(0): S– Bucket(1): M– Bucket(2): L
Monitor v$sql_cs_statistics.rows_processed
Query :b1 :b2 OptimalRows
Processedq1 9 33 N1/N2 37,382
q2 5 32 N1/N2 2
q3 2 999 FTS/FTS 8,021,324
q4 9 999 N1/FTS 6,233,815
q5 2 33 FTS/N2 1,825,131
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ACS Demo
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Demo 6: When Cursor becomes Bind Aware?
Query :b1 :b2 OptimalRows
Processed Bucket Aware Child Actualq1 9 33 N1/N2 37,382
q2 5 32 N1/N2 2
q3 2 999 FTS/FTS 8,021,324
q4 9 999 N1/FTS 6,233,815
q5 2 33 FTS/N2 1,825,131
Obtain rows processed from demo 1-5 then guesstimate aware flag
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Demo 6: When Cursor becomes Bind Aware?
Query :b1 :b2 OptimalRows
Processed Bucket Aware Child Actualq1 9 33 N1/N2 37,382 1
q2 5 32 N1/N2 2 0
q3 2 999 FTS/FTS 8,021,324 2
q4 9 999 N1/FTS 6,233,815 2
q5 2 33 FTS/N2 1,825,131 2
Obtain rows processed from demo 1-5 then guesstimate aware flag
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Demo 6: When Cursor becomes Bind Aware?
Query :b1 :b2 OptimalRows
Processed Bucket Aware Child Actualq1 9 33 N1/N2 37,382 1 N
q2 5 32 N1/N2 2 0 N
q3 2 999 FTS/FTS 8,021,324 2 Y
q4 9 999 N1/FTS 6,233,815 2 Y
q5 2 33 FTS/N2 1,825,131 2 Y
Obtain rows processed from demo 1-5 then guesstimate aware flag
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Demo 6: When Cursor becomes Bind Aware?
Query :b1 :b2 OptimalRows
Processed Bucket Aware Child Actualq1 9 33 N1/N2 37,382 1 N 0 N1/N2
q2 5 32 N1/N2 2 0 N 0 N1/N2
q3 2 999 FTS/FTS 8,021,324 2 Y 1 FTS/FTS
q4 9 999 N1/FTS 6,233,815 2 Y 2 N1/FTS
q5 2 33 FTS/N2 1,825,131 2 Y 3 FTS/N2
Obtain rows processed from demo 1-5 then guesstimate aware flag
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Demo 6: When Cursor becomes Bind Aware?
Query :b1 :b2 OptimalRows
Processed Bucket Aware Child Actualq1 9 33 N1/N2 37,382 1 N 0 N1/N2
q2 5 32 N1/N2 2 0 N 0 N1/N2
q3 2 999 FTS/FTS 8,021,324 2 Y 1 FTS/FTS
q4 9 999 N1/FTS 6,233,815 2 Y 2 N1/FTS
q5 2 33 FTS/N2 1,825,131 2 Y 3 FTS/N2
Obtain rows processed from demo 1-5 then guesstimate aware flag
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Demo 7: When Cursor becomes Bind Aware?
Query :b1 :b2 OptimalRows
Processed Bucket Aware Child Actualq5 2 33 FTS/N2 1,825,131
q4 9 999 N1/FTS 6,233,815
q3 2 999 FTS/FTS 8,021,324
q2 5 32 N1/N2 2
q1 9 33 N1/N2 37,382
Compute bucket and guesstimate aware flag and actual plan
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Demo 7: When Cursor becomes Bind Aware?
Query :b1 :b2 OptimalRows
Processed Bucket Aware Child Actualq5 2 33 FTS/N2 1,825,131 2
q4 9 999 N1/FTS 6,233,815 2
q3 2 999 FTS/FTS 8,021,324 2
q2 5 32 N1/N2 2 0
q1 9 33 N1/N2 37,382 1
Compute bucket and guesstimate aware flag and actual plan
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Demo 7: When Cursor becomes Bind Aware?
Query :b1 :b2 OptimalRows
Processed Bucket Aware Child Actualq5 2 33 FTS/N2 1,825,131 2 N
q4 9 999 N1/FTS 6,233,815 2 N
q3 2 999 FTS/FTS 8,021,324 2 N
q2 5 32 N1/N2 2 0 N
q1 9 33 N1/N2 37,382 1 Y
Compute bucket and guesstimate aware flag and actual plan
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Demo 7: When Cursor becomes Bind Aware?
Query :b1 :b2 OptimalRows
Processed Bucket Aware Child Actualq5 2 33 FTS/N2 1,825,131 2 N 0 FTS/N2
q4 9 999 N1/FTS 6,233,815 2 N 0 FTS/N2
q3 2 999 FTS/FTS 8,021,324 2 N 0 FTS/N2
q2 5 32 N1/N2 2 0 N 0 FTS/N2
q1 9 33 N1/N2 37,382 1 Y 1 N1/N2
Compute bucket and guesstimate aware flag and actual plan
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Demo 7: When Cursor becomes Bind Aware?
Query :b1 :b2 OptimalRows
Processed Bucket Aware Child Actualq5 2 33 FTS/N2 1,825,131 2 N 0 FTS/N2
q4 9 999 N1/FTS 6,233,815 2 N 0 FTS/N2
q3 2 999 FTS/FTS 8,021,324 2 N 0 FTS/N2
q2 5 32 N1/N2 2 0 N 0 FTS/N2
q1 9 33 N1/N2 37,382 1 Y 1 N1/N2
Compute bucket and guesstimate aware flag and actual plan
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Demo 8: When Cursor becomes Bind Aware?
Query :b1 :b2 OptimalRows
Processed Bucket Aware Child Actualq5 2 33 FTS/N2 1,825,131 2
q4 9 999 N1/FTS 6,233,815 2
q3 2 999 FTS/FTS 8,021,324 2
q1 9 33 N1/N2 37,382 1
q2 5 32 N1/N2 2 0
Guesstimate aware flag and actual plan
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Demo 8: When Cursor becomes Bind Aware?
Query :b1 :b2 OptimalRows
Processed Bucket Aware Child Actualq5 2 33 FTS/N2 1,825,131 2 N
q4 9 999 N1/FTS 6,233,815 2 N
q3 2 999 FTS/FTS 8,021,324 2 N
q1 9 33 N1/N2 37,382 1 N
q2 5 32 N1/N2 2 0 N
Guesstimate aware flag and actual plan
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Demo 8: When Cursor becomes Bind Aware?
Query :b1 :b2 OptimalRows
Processed Bucket Aware Child Actualq5 2 33 FTS/N2 1,825,131 2 N 0 FTS/N2
q4 9 999 N1/FTS 6,233,815 2 N 0 FTS/N2
q3 2 999 FTS/FTS 8,021,324 2 N 0 FTS/N2
q1 9 33 N1/N2 37,382 1 N 0 FTS/N2
q2 5 32 N1/N2 2 0 N 0 FTS/N2
Guesstimate aware flag and actual plan
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Demo 8: When Cursor becomes Bind Aware?
Query :b1 :b2 OptimalRows
Processed Bucket Aware Child Actualq5 2 33 FTS/N2 1,825,131 2 N 0 FTS/N2
q4 9 999 N1/FTS 6,233,815 2 N 0 FTS/N2
q3 2 999 FTS/FTS 8,021,324 2 N 0 FTS/N2
q1 9 33 N1/N2 37,382 1 N 0 FTS/N2
q2 5 32 N1/N2 2 0 N 0 FTS/N2
Guesstimate aware flag and actual plan
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Closing Remarks
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Remarks on Bind Sensitivity
Monitor V$SQL_CS_STATISTICS.rows_processed– If small number of rows then
V$SQL_CS_HISTOGRAM.bucket_id(0)++– If medium number of rows then
V$SQL_CS_HISTOGRAM.bucket_id(1)++– If large number of rows then
V$SQL_CS_HISTOGRAM.bucket_id(2)++
Based on experimental observation
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Remarks on Bind Aware
Some cases where cursor may become bind aware– bucket_id(0) = bucket_id(1) > 0– bucket_id(1) = bucket_id(2) > 0– bucket_id(0) > 0 and bucket_id(2) > 0– /*+ BIND_AWARE */ CBO Hint
Based on experimental observation
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Understanding Selectivity ProfileFrom demo 6
Query :b1 :b2 Childq3 2 999 1
q4 9 999 2
q5 2 33 3
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Understanding Selectivity ProfileFrom demo 6
Query :b1 :b2 Childq3 2 999 1
q4 9 999 2
q5 2 33 3
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Understanding Selectivity ProfileFrom demo 6
Query :b1 :b2 Childq3 2 999 1
q4 9 999 2
q5 2 33 3
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Conclusions
ACS only applies to a subset of queries with binds ACS requires a ramp-up process (few executions) In some cases cursor may fail to become bind aware To force a cursor become bind aware use CBO Hint ACS is not persistent ACS works well with SQL Plan Management
ACS can produce multiple optimal plans for one query
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Give Away
Creates a SQL Patch for one SQL_ID Turns “on” EVENT 10053 for SQL_ID Hints on SQL Patch
– GATHER_PLAN_STATISTICS – MONITOR – BIND_AWARE
Instructions how to drop SQL Patch and turn “off” EVENT 10053
Script sqlt/utl/coe_gen_sql_patch.sql (MOS 215187.1)
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References and Contact Info
https://blogs.oracle.com/optimizer/– Insight into the workings of the Optimizer
Oracle Optimizer Blog
[email protected] http://carlos-sierra.net @csierra_usa
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