How to Read an Execution Plan

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1 How to Read an Execution Plan Chris Saxon, @ChrisRSaxon & @SQLDaily blogs.oracle.com/sql youtube.com/c/TheMagicofSQL asktom.oracle.com Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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someone confused/frustrated – how does that help me?! How the #@%$ does that help? Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon Ryan McGuire / Gratisography

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-------------------------------------- | Id | Operation | Name | -------------------------------------- | 0 | SELECT STATEMENT | | | 1 | HASH JOIN | | | 2 | TABLE ACCESS FULL| COLOURS | | 3 | TABLE ACCESS FULL| BRICKS | -------------------------------------- Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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someone confused/frustrated – how does that help me?! Those don't look like directions… Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon Ryan McGuire / Gratisography

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12 How to Read an Execution Plan Chris Saxon, @ChrisRSaxon & @SQLDaily blogs.oracle.com/sql youtube.com/c/TheMagicofSQL asktom.oracle.com Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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-------------------------------------- | Id | Operation | Name | -------------------------------------- | 0 |SELECT STATEMENT | | | 1 |HASH JOIN | | | 2 |TABLE ACCESS FULL| COLOURS | | 3 |TABLE ACCESS FULL| BRICKS | -------------------------------------- Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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Following an Execution Plan Go to the first unvisited leaf Go to its parent Repeat! 1 2 3 Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon Depth-first search!

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-------------------------------------- | Id | Operation | Name | -------------------------------------- | 0 | SELECT STATEMENT | | | 1 | SORT AGGREGATE | | | 2 | TABLE ACCESS FULL| BRICKS | | 3 | TABLE ACCESS FULL | COLOURS | -------------------------------------- Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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select ( select count(*) from bricks b where b.colour = c.colour ) brick# from colours c; Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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someone confused/frustrated – how does that help me?! But how do I make it faster?! Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon Ryan McGuire / Gratisography

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Plan Tuning Questions Row Estimates Are they accurate? I/O Reads How much work did we do? Access Methods Can we read the rows more efficiently? Step Duration How long did each operation take? Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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DBMS_XPlan set serveroutput off select /*+ gather_plan_statistics */... from ...; select * from table ( dbms_xplan.display_cursor ( null, null, 'ALLSTATS LAST' ) ); Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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--------------------------------------------------- | Starts | E-Rows | A-Rows | A-Time | Buffers | --------------------------------------------------- | 1 | | 56 |00:00:00.01 | 24 | | 1 | 68 | 56 |00:00:00.01 | 24 | | 1 | 8 | 7 |00:00:00.01 | 18 | | 1 | 4 | 3 |00:00:00.01 | 12 | | 1 | 3 | 3 |00:00:00.01 | 6 | | 1 | 5 | 5 |00:00:00.01 | 6 | | 1 | 9 | 9 |00:00:00.01 | 6 | | 1 | 24 | 24 |00:00:00.01 | 6 | Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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--------------------------------------------------------------- | Operation | Name | Starts | E-Rows | A-Rows | --------------------------------------------------------------- | SELECT STATEMENT | | 1 | | 56 | | HASH JOIN | | 1 | 94 | 56 | | HASH JOIN | | 1 | 42 | 24 | | MERGE JOIN CARTESIAN| | 1 | 5 | 120 | | TABLE ACCESS FULL | BRICKS | 1 | 1 | 24 | | BUFFER SORT | | 24 | 5 | 120 | | TABLE ACCESS FULL | TOYS | 1 | 5 | 5 | | TABLE ACCESS FULL | COLOURS | 1 | 100 | 3 | | TABLE ACCESS FULL | PENS | 1 | 9 | 9 | --------------------------------------------------------------- Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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begin dbms_stats.gather_table_stats ( user, 'bricks' ); dbms_stats.gather_table_stats ( user, 'toys' ); end; / Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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------------------------------------------------------------- | Operation | Name | Starts | E-Rows | A-Rows | ------------------------------------------------------------- | SELECT STATEMENT | | 1 | | 56 | | HASH JOIN | | 1 | 68 | 56 | | HASH JOIN | | 1 | 8 | 7 | | HASH JOIN | | 1 | 4 | 3 | | TABLE ACCESS FULL| COLOURS | 1 | 3 | 3 | | TABLE ACCESS FULL| TOYS | 1 | 5 | 5 | | TABLE ACCESS FULL | PENS | 1 | 9 | 9 | | TABLE ACCESS FULL | BRICKS | 1 | 24 | 24 | ------------------------------------------------------------- Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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select count (*) from bricks where colour = 'red' and shape = 'cylinder'; Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon select count (*) from bricks where colour = 'red' and shape = 'prism';

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------------------- | E-Rows | A-Rows | ------------------- | | 1 | | 1 | 1 | | 3 | | ------------------- Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon ------------------- | E-Rows | A-Rows | ------------------- | | 1 | | 1 | 1 | | 3 | | -------------------

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------------------- | E-Rows | A-Rows | ------------------- | | 1 | | 1 | 1 | | 3 | 8 | ------------------- Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon ------------------- | E-Rows | A-Rows | ------------------- | | 1 | | 1 | 1 | | 3 | 0 | -------------------

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begin dbms_stats.gather_table_stats ( user, 'bricks', method_opt => ' for columns ( colour, shape )' ); end; / Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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------------------- | E-Rows | A-Rows | ------------------- | | 1 | | 1 | 1 | | 8 | 8 | ------------------- Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon ------------------- | E-Rows | A-Rows | ------------------- | | 1 | | 1 | 1 | | 3 | 0 | -------------------

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------------------- | E-Rows | A-Rows | ------------------- | | 1 | | 1 | 1 | | 1 | 8 | ------------------- Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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begin dbms_stats.gather_table_stats ( user, 'bricks', method_opt => ' for columns ( upper ( colour ) )' ); end; / Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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--------------------------------------------------- | Operation | Name | A-Rows | Buffers | --------------------------------------------------- | SELECT STATEMENT | | 1 | 13 | | HASH JOIN | | 1 | 13 | | TABLE ACCESS FULL| BRICKS | 1 | 7 | | TABLE ACCESS FULL| COLOURS | 1 | 6 | --------------------------------------------------- Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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alter table bricks add constraint bricks_pk primary key ( brick_id ); create index colour_i on colours ( colour ); Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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--------------------------------------------------- | Operation | Name | Buffers | --------------------------------------------------- | SELECT STATEMENT | | 4 | | NESTED LOOPS | | 4 | | TABLE ACCESS BY INDEX … | BRICKS | 2 | | INDEX UNIQUE SCAN | BRICKS_PK | 1 | | TABLE ACCESS BY INDEX … | COLOURS | 2 | | INDEX RANGE SCAN | COLOUR_I | 1 | --------------------------------------------------- Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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--------------------------------------------------- | Operation | Name | Starts | Buffers | --------------------------------------------------- | SELECT STATEMENT | | 1 | 6 | | SORT AGGREGATE | | 3 | 18 | | TABLE ACCESS FULL| BRICKS | 3 | 18 | | TABLE ACCESS FULL | COLOURS | 1 | 6 | --------------------------------------------------- Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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select count(*) brick# from colours c left join bricks b on b.colour = c.colour group by c.colour; Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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---------------------------------------------------- | Operation | Name | Starts | Buffers | ---------------------------------------------------- | SELECT STATEMENT | | 1 | 12 | | HASH GROUP BY | | 1 | 12 | | HASH JOIN OUTER | | 1 | 12 | | TABLE ACCESS FULL| COLOURS | 1 | 6 | | TABLE ACCESS FULL| BRICKS | 1 | 6 | ---------------------------------------------------- Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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------------------------------------------------- | Operation | Name | Starts | A-Rows | ------------------------------------------------- | SELECT STATEMENT | | 1 | 3 | | HASH GROUP BY | | 1 | 3 | | TABLE ACCESS FULL| BRICKS | 1 | 100K| ------------------------------------------------- Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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create materialized view brick_mv enable query rewrite as select colour, count (*) from bricks group by colour; Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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------------------------------------------------------------- | Operation | Name | Starts | A-Rows | ------------------------------------------------------------- | SELECT STATEMENT | | 1 | 3 | | MAT_VIEW REWRITE ACCESS FULL| BRICK_MV | 1 | 3 | ------------------------------------------------------------- Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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Getting Better Plans Check row estimates Create faster access paths Review requirements! 1 2 3 Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon

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Where do I go from here? Learn how to get a plan Get the scripts Watch the videos 1 2 3 https://bit.ly/exec-plan https://bit.ly/plan-scripts Copyright © 2020 Oracle and/or its affiliates. blogs.oracle.com/sql www.youtube.com/c/TheMagicOfSQL @ChrisRSaxon https://bit.ly/sql-performance-videos

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