IBM 000-8697 Manual de usuario descargar pdf (Pagina 328)

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3-72 IBM Informix OnLine Database Server Administrator’s Guide

Monitor Chunks

Executetbcheck -pr to obtainthechunkinformationthat is storedinthe root

dbspace reserved page. Referto page 2-100 for adetailed descriptionof each

ﬁeld in the chunk reserved page, PCHUNK.

Execute tbcheck -pe to obtain the physical layout of information in the

chunk.Thechunklayoutissequential,andthenumberofpagesdedicatedto

each table is shown. The following information displays:

■ Dbspace name, owner, and number

■ Number of chunks in the dbspace

This output is useful for determining the extent of chunk fragmentation. If

OnLineisunabletoallocateanextentinachunkdespiteanadequatenumber

of free pages, the chunk might be badly fragmented.

Refer to page 7-43 for further information about the tbcheck -pe output.

Depending on the speciﬁc circumstances, you might be able to eliminate

fragmentationby using theALTER TABLE statementtorebuildthetables. For

this tactic to work, the chunk must contain adequate contiguous space in

which to rebuild each table. In addition, the contiguous space in the chunk

must be the space that OnLine normally allocates to rebuild the table. (That

is, OnLine allocates space for the ALTER TABLE processing from the

beginning ofthe chunk, looking for blocksof free spacethat are greater than

or equal to the size speciﬁedfor theNEXT EXTENT. Ifthe contiguousspace is

located near the end of the chunk, OnLine could rebuild the table using

blocks of space that are scattered throughout the chunk.)

Use the ALTER TABLE statement on every table in the chunk. Follow these

steps:

1. For each table, drop all the indexes except one.

2. Cluster the remaining index using the ALTER TABLE statement.

3. Re-create all the other indexes.

You eliminate the fragmentation in the second step, when you rebuild the

table by rearranging the rows. In the third step, you compact the indexes as

wellbecause the index valuesaresorted beforethey areadded to the B+tree.

You donot need todrop any ofthe indexes before youcluster one but,if you

do, the ALTER TABLE processing is faster and you gain the beneﬁt of more

compact indexes.

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