Guidelinesi li for Modifyingi i a Sequence

•• You mustt be thet ownerr orr have thet

ALTER privilegeri il forf r thet sequence..

•• Onlyly futuref t re sequence numbersrs arere affectedff t ..

•• The sequence mustt be droppedr and rere--createdr t toto restartr t rt thet sequence att a differentiff r t numberr..

•• Some validationli ti isis performedrf r ..

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Removingi a Sequence

•• Remove a sequence fromfr thet datata dictionaryi ti ry by usingi thet DROP SEQUENCE statementt t t..

•• Once removed,r , thet sequence can no longerl r be referencedr f r ..

SQL> DROP SEQUENCE dept_deptno;

Sequence dropped.

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What IsIs an Index?I

•• Schema objectj t

•• Used by thet Oracler le Serverr r toto speed up thet retrievalr tri l off rowsr by usingi a pointeri t r

•• Reduces diski I/OI/O by usingi rapidr id pathth access methodt toto locatel te thet datata quicklyi ly

•• IndependentI t off thet tablet le itit indexesi

•• Automaticallyt ti lly used and maintainedi t i by thet Oracler le Serverr r

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How Are IndexesI Created?

•• Automaticallyt ti lly

–A uniquei indexi isis createdr t automaticallyt ti lly when you definefi a PRIMARYI KEY orr UNIQUEI key constrainttr i t inin a tablet le definitionfi iti ..

•• Manuallylly

–Usersrs can creater te nonuniquei indexesi

on columnsl toto speed up access timeti toto thet rowsr ..

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Creatingi an IndexI

•• Creater te an indexi on one orr morere columnsl

CREATE INDEX index

ON table (column[, column]...);

•• ImproveI r thet speed off queryry access on thet ENAME columnl inin thet EMP tablet le

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Guidelinesi li to Creatingi an IndexI

•• The columnl isis used frequentlyfre tly inin thet WHERE clausel orr inin a joinj in conditioniti ..

•• The columnl containst i a widei ranger off valuesl ..

•• The columnl containst i a largel r numberr off nullll valuesl ..

•• Two orr morere columnsl arere frequentlyfre tly used togethert t r inin a WHERE clausel orr a joinj in conditioniti ..

•• The tablet le isis largel r and mostt queriesri arere expectedt toto retriever tri lessl thant 2–4% off thet rowsr ..

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Guidelinesi li to Creatingi an IndexI

Do not create an index if:

•• The tablet le isis smallll

•• The columnsl arere nott oftenft used as a conditioniti inin thet queryry

•• Mostt queriesri arere expectedt toto retriever tri morere thant 2–4% off thet rowsr

•• The tablet le isis updatedt frequentlyfre tly

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Confirmingi i IndexesI

•• The USER_INDEXESI datata dictionaryi ti ry viewi containst i thet name off thet indexi and itsits uniquenessi ..

•• The USER_INDI _COLUMNS viewi containst i thet indexi name,, thet tablet le name,, and thet columnl name..

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Removingi an IndexI

•• Remove an indexi fromfr thet datata dictionaryi ti ry..

SQL> DROP INDEX index;

•• Remove thet EMP_ENAME_IDXI indexi fromfr thet datata dictionaryi ti ry..

SQL> DROP INDEX emp_ename_idx;

Index dropped.

•• To dropr an index,i , you mustt be thet ownerr off thet indexi orr have thet DROPANY INDEXI privilegeri il ..

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Synonyms

Simplify access to objects by creating a synonym (another name for an object).

•• Referf r toto a tablet le owned by anothert r userr..

•• Shortenrt lengthyl t objectj t names..

CREATE [PUBLIC] SYNONYM synonym

FOR object;

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