0oracle對象定義-概念(45)-資料下載頁
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【正文】 pports multiple transactions ? Each transaction writes to its own block(s) ? If extent fills, automatically extends ? If all extents fill, segment wraps ? If segment wraps, undo data overwritten ? If undo data not available, error occurs Copyright 169。 2023 Global Business Solutions, Inc. Read Consistency ? When query executes: ? Current time determined ? Blocks with same time read ? Newer blocks reconstructed with rollback segments Copyright 169。 2023 Global Business Solutions, Inc. Temporary Segments ? Provide temporary workspace ? Created automatically for joins, indexes, sorts ? Created on disk when memory full ? Sized by DEFAULT STORAGE clause of tablespace ? Reclaimed by SMON Bootstrap Segment Rollback Segment Index Segment Temporary Segment Copyright 169。 2023 Global Business Solutions, Inc. Bootstrap Segment ? Contains dictionary definitions loaded on open ? Is not user accessible ? Resides in SYSTEM tablespace ? Requires no DBA maintenance Bootstrap Segment Rollback Segment Index Segment Bootstrap Segment Copyright 169。 2023 Global Business Solutions, Inc. Database Extents ? Consist of a set of contiguous blocks ? Serve as space allocation for segment ? Deallocated when segment is removed ? First extent created as Initial Extent ? Grow by Next Extent Segment Extent Extent Extent Copyright 169。 2023 Global Business Solutions, Inc. Database Blocks ? Are: ? smallest unit of storage ? sized by DB_BLOCK_SIZE ? same as buffer in database buffer cache ? Correspond to O/S block ? Typically sized as 2k, 4k, 8k, or 16k Segment Extent Blocks Copyright 169。 2023 Global Business Solutions, Inc. Define Database Objects Summary ? Tablespaces are logical collections of database objects with one or more datafiles ? Upon creation, space is preallocated for tablespaces Segment Extent Blocks Copyright 169。 2023 Global Business Solutions, Inc. Define Database Objects Summary ? Segments are preallocated when objects created ? Segments consist of extents ? Extents are a set of contiguous blocks ? As data is added, segments fill up ? Storage management is essential Data Segment Copyright 169。 2023 Global Business Solutions, Inc. 演講完畢，謝謝觀看！
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