IBM 000-8697 Instrukcja Użytkownika Strona 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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