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When you create a table or index, you can usually set an attribute to control how much free space a page should contain initially This attribute goes by various names For example, Oracle calls it PCTFREE (percent to leave free), while Ingres calls it FILLFACTOR (percent to fill) Table 8-4 shows the PCTFREE equivalent for the Big Eight Table 8-4 DBMSs and Free Space Attribute PCTFREE Equivalent PCTFREE FILLFACTOR FILLFACTOR No support FILLFACTOR No support PCTFREE FILLFACTOR
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IBM Informix Ingres InterBase Microsoft MySQL Oracle Sybase
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Consider how Oracle does an INSERT 1 Calculate size of new row 2 Find the "last" page (This is for the simplest case, a heap When Oracle is adding to a heap, the DBMS is looking at pages in the last extent or the freelist) 3 Look at the page header to find out how much space is available
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4 If (available space < PCTFREE)
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If (there are no more pages) Allocate new extent Goto Step #2
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8 Copy new row into page, update page header, and stop Given that PCTFREE just causes space to be wasted in every page, why would you want a PCTFREE greater than zero That's easy: to avoid migration If free space is in a page, then an expanding UPDATE will just grow into it, rather than migrating It's generally accepted that migration is such an awful thing that you should be willing to accept 10% waste space in every page in return for reduced migration Therefore, for example, Oracle's default PCTFREE setting is 10% that is, Oracle reserves 10% of each page for updates to existing rows, by default Hold on, though Remember that migration happens only if there is an expanding UPDATE But expanding UPDATEs can't happen if (a) the table is read-only or (b) all the table's columns are fixedsize and there is no plan to ALTER TABLE to amend the column sizes Under either of these conditions, you should override the default and set PCTFREE to equal zero (or FILLFACTOR to equal 100) Incidentally, if a table is read-only, then it's also good practice to use larger page sizes Incidentally (again), if all columns are fixed-size, then the amount of per-page overhead space is slightly less Remember that a column is not truly fixed-size unless it's defined as NOT NULL, and remember too that InterBase and Oracle use variable-length for almost every situation You can dispense with PCTFREE entirely if you allocate free space in selected rows at INSERT time that is, by putting a filler in variable-length columns as suggested in 7, "Columns" The bottom line is: avoid migration Here's how:
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Add a bit of filler when you do the original INSERT Use PCTFREE (or its equivalent) Don't do DELETE plus INSERT Do UPDATE instead Put DELETEs and shrinking UPDATEs before expanding UPDATEs or INSERTs Arrange batches in ROWID order If columns will be changed regularly, define them as fixed-size fields, not variable-length fields
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The Bottom Line: Heaps
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Most DBMSs have a row identifier that you can use in SQL statements While using ROWID allows you to write the fastest possible SELECT, ROWID also poses two dangers The first is that another user could DELETE the row, which makes the row identifier invalid The second, and greater, danger is that another user could DELETE the row and then INSERT a new row at the same location, which makes the row identifier a valid, but false, pointer The only completely safe use of ROWID is inside a serialized transaction The process of finding a home for an expanding UPDATE is called migration Migration causes slow access due to pointers It also wastes space A group of pages is fragmented if (a) excess free space is on many pages because of deletions, shrinking UPDATEs, or migrations, (b) the logical order of rows is not the same as the physical
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arrangement because of pointers left by migrations, or (c) pages or extents for different tables are interleaved Fragmentation wastes space and costs time The DBMS can try to reduce fragmentation automatically by reclaiming wasted space But it is not possible to stop fragmentation, and eventually you must rebuild the database or run a utility to reorganize the pages When you create a table or index, you can usually set an attribute to control how much free space a page should contain initially Use this feature to avoid migration, which is a larger problem than wasted space Because expanding UPDATEs can't happen if (a) the table is read-only or (b) all the table's columns are fixed-size, set PCTFREE to zero (or FILLFACTOR to 100) in these cases If a table is read-only, it's good practice to use larger page sizes
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