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BradleyMickel

LearnhowtoAnalyze&TuneMySQLQueriesforBetterPerformanceToolsandtechniquesforfasterqueries

MySQLDBAThursdayJune21st

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ToolsOverviewofcommontoolsusedtoidentifyqueryoptimizations

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EXPLAIN

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EXPLAINSELECT*FROMstaff;

EXPLAINSELECT*FROMstaff\G

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JSONFORMATEXPLAIN

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EXPLAINFORMAT=JSONSELECTcount(1)FROMsakila.rentalWHERElast_update>‘2006-02-1800:00:00'\G

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EXPLAINFields

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Field JSONFIELD Descriptionid select_id ThesequentialnumberoftheSELECTwithinthequeryselect_type TheTypeofselectbeingperformed.table table_name Thename,oraliasofthetablerelatedtothisstepofthequeries

partitions partitions Thepartitionsfromwhichrecordswouldbematchedbythequery.NULLfornonpartitionedtables

type access_type Thetypeofaccessbeingusedtoretrievedataorjointablespossible_keys possible_keys Whichindexesarebeingreviewedforuseinthequerykey key Theindexwhichwillbeusedforthequery

key_len key_length ThelengthofthekeythatMySQLhasdecidedtouse.Thisenablesyoutodeterminehowmanypartsofacompositeindexarebeingused.*

ref ref Shows which columns or constants are compared to the index named in the key column to select rows from the table

rows rows The number of rows MySQL estimates will be need to be reviewedfiltered filtered Anestimatedpercentageoftablerowsthatwillbefilteredbythecondition.100%meansall

therowsreturnedbytheconditionwillbejoinedwithprevioustablesextra Additionalinformationrelatedtotheoptimizer’schosenpath.

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JSONFORMATEXPLAIN

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▪AdditionalFields:• cost_Info

• query_cost• used_key_parts

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JSONFORMATEXPLAIN

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▪AdditionalFields:• rows_examined_per_scan• rows_examined_per_join• cost_info

• read_cost• eval_cost• prefix_cost• data_read_per_join

• used_columns

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AccessTypes

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Type Descriptionconstant Thetablehasatmostonematchingrowwhichisreadatthestartofthequerysystem Atypeofconstanttableusedforsystemstableseq_ref Usedfortablejoinswitha1:1relationship.RequiresthejoiningindexbyaPrimaryorUniqueKey

ref Allrowswithmatchingindexvaluesarereadfromthistableforeachcombinationofrowsfromtheprevioustables

fulltext AfulltextIndexisbeingusedref_or_null Similartoref,butwillfindnullvaluesinadditiontotheselectedvalueindex_merge IndexMergeisbeingusedforthejoinunique_subquery Replaceseq_refforcertainsubqueriesusingINindex_subquery Similartounique_subquerybutforwhentheindexdoesnothaveauniqueconstraint

rangeOnly rows that are in a given range are retrieved, using an index to select the rows. The key column in the output row indicates which index is used. The key_len contains the longest key part that was used. The ref column is NULL for this type.

index Theentireindexisscannedall Afulltablescanisrequired

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PerformanceSchemaProfilingSELECTevent_id,sql_textFROMperformance_schema.events_statements_history_longWHEREsql_textlike‘%select*fromcity%’;

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PerformanceSchemaProfilingcont.

SELECTevent_nameASStage,round(timer_wait/pow(10,12),6)ASDurationFROMevents_stages_history_longWHEREnesting_event_id=54;

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useperformance_schema;

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Pt-query-digest

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▪AnalysesMySQLqueriesfromtheslow,general,andbinarylogs▪Groupsqueriesbyfingerprint▪providesaverageruntimeinformation

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IndexesIndexconceptsandlimitations

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BalancedTreeIndex

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CompositeBalancedTreeIndex

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PrimaryKeyRequirementsandconstraints▪Field(s)MUSTbeUNIQUE▪NULLisnotvalid▪Only1pertable▪Canbeasingleormultiplefields

Controlshowdataisstoredphysically

IsutilizedaspartofALLsecondaryindexes�15

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IndexImpactSELECTSQL_NO_CACHEcount(1)FROMsakila.rentalWHERElast_update>'2006-02-1800:00:00'\G

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IndexImpactcontSELECTcount(1)FROMsakila.rentalWHERElast_update>'2006-02-1800:00:00'\G

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MultipleTablesSELECTf.rating,sum(p.amount)asincomeFROMfilmfJOINinventoryiONi.film_id=f.film_idJOINrentalrONr.inventory_id=i.inventory_idJOINpaymentpONp.rental_id=r.rental_idGROUPBYf.ratingORDERBYincomeDESC;

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MultipleTablesSELECTf.rating,sum(p.amount)asincomeFROMfilmfJOINinventoryiONi.film_id=f.film_idJOINrentalrONr.inventory_id=i.inventory_idJOINpaymentpONp.rental_id=r.rental_idGROUPBYf.ratingORDERBYincomeDESC;

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IndexLimitationsFunctions▪Aggregates

• SUM,AVG• DoesnotincludeMIN,MAX▪MultipleRangescans

• multipleORstatements• multiplelessthanorgreaterthan▪CharacterSetDifferences▪DataTypeDifferences

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MultipleRangeScansSELECTcount(1)FROMrentalWHEREcustomer_id=148ANDreturn_dateBETWEEN'2005-08-2900:00:00'AND'2005-09-0523:59:59'ANDlast_updateBETWEEN'2006-02-1500:00:00'AND'2006-02-1523:59:59';

Addacompositeindexoncustomer_id,return_date,last_update

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MultipleRangeScans

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MultipleRangeScansSELECTCOUNT(1)FROMrentalWHEREcustomer_id=148AND(return_dateBETWEEN'2005-08-2900:00:00'AND'2005-09-0523:59:59'ORlast_updateBETWEEN'2006-02-1500:00:00'AND'2006-02-1523:59:59');

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MultipleRangeScansSELECTCOUNT(1)FROMrentalWHEREcustomer_id=148ANDreturn_dateBETWEEN'2005-08-2900:00:00’AND'2005-09-0523:59:59'UNIONSELECTCOUNT(1)FROMrentalWHEREcustomer_id=148ANDlast_updateBETWEEN'2006-02-1500:00:00'AND'2006-02-1523:59:59';

Indexesarereadlefttorightsoanewindexisneededoncustomer_id,last_update

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MultipleRangeScans

OR

UNION

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Referenceshttps://dev.mysql.com/doc/sakila/en/sakila-installation.htmlhttps://dev.mysql.com/doc/refman/5.7/en/explain-output.html#explain-join-typeshttps://dev.mysql.com/doc/mysql-perfschema-excerpt/5.7/en/performance-schema-query-profiling.htmlhttps://dev.mysql.com/doc/refman/5.7/en/general-thread-states.htmlhttps://www.cs.usfca.edu/~galles/visualization/BPlusTree.htmlhttps://www.percona.com/software/database-tools/percona-toolkithttps://bugs.mysql.com/bug.php?id=83062