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In this section, we'll look at a particular storage structure called a heap A heap, or heap-organized table, is a structure for storing data in an unstructured manner When you add something to a heap, it goes wherever free space is available, which probably means "at the end" existing data is not moved to make free space available for new data Heaps are the default structure They're certainly the simplest type
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Here is a nonstandard Oracle SQL-extension statement that seeks out a particular data row:
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SELECT column1 FROM Table1 WHERE ROWID = '00001F2000A30001' FOR UPDATE OF column1
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In this statement, the ROWID value has three parts: (a) the page number within a file, (b) the row number within the page, and (c) the file number within the list of database files that Oracle keeps in the system catalog In other words, the Oracle ROWID is a row identifier it uniquely describes a row of a heap-organized table to the DBMS Other DBMSs have different formats and use different terms, as shown in Table 8-3 But the name doesn't matter the important thing is that most DBMSs have a row identifier that you can use in SQL statements, and we'll call this the "ROWID" throughout this book The question is Should you The Oracle statement shown earlier is the fastest possible SELECT With any other column, the DBMS has to take the column value and find the address by looking it up With a ROWID, the column value is the address Therefore, one way to go through a table is like this:
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SELECT ROWID FROM Table1 SELECT * FROM Table1 WHERE ROWID =
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The first SELECT statement gets all the row identifiers into a program buffer so that you can then access each individual row with the second SELECT Using ROWID speeds up SELECT On the other hand, the Oracle statement can also fail or can cause an error to appear in the database The obvious danger is that another user could delete the row, which makes the row identifier invalid The greater danger is that another user could delete the row, then INSERT a new row at the same location which makes the row identifier a valid, but false, pointer Some further dangers are specific to the DBMS or table type you're using:
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Informix won't support true row identifiers if there's partitioning Table 8-3 DBMSs and ROWID
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IBM Informix Ingres InterBase Microsoft MySQL Oracle Sybase
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ROWID Equivalent RID ROWID tid No support RID _rowid ROWID RID
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Microsoft will let rows change location if there's a clustered index PostgreSQL doesn't document how its row identifiers work so if something goes wrong you can't hold them responsible
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This is hardly surprising to anyone who understands relational theory The fact is that row identifiers are not an SQL feature they are a trap door for getting away from SQL They are useful if they weren't then DBMSs wouldn't allow them but the only completely safe use of ROWID is inside a serialized transaction (A serialized transaction is a transaction that prevents or avoids data changes by other users, and you shouldn't use one until you've read 15, "Locks") Inside a transaction, row identifiers can be used for navigation for example, to simulate subqueries and outer joins with application program code when the DBMS doesn't support such advanced features Incidentally, row identifiers are "pseudocolumns" rather than real columns They take up no space in the row They do not appear if you say SELECT * because SELECT * only gets defined columns You must specify row identifiers explicitly to get them
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We saw in the last chapter that variable-length columns are common Even if you think you use fixedsize data types, there's no way to avoid variable-length columns that works with all DBMSs Suppose you have a situation in which Row #1 on Page #1 contains one variable-length column with value NULL This UPDATE is executed:
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UPDATE Table1 SET column1 = 'abcdefg'
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This is called an expanding UPDATE because the row is clearly getting bigger This is a problem because no free space is between Row #1 and Row #2 on the same page (because column1 is variable-length) Therefore, before the DBMS can modify Row #1, it must shift Row #2 down Incidentally, because shifting takes time, it takes more time to update variable-length columns if you use large (16KB or more) page sizes But that's not our immediate concern Our real concern is What if Page #1 is already full Pages are fixed-size the DBMS can't make them bigger And the DBMS can't simply shift rows forward in all subsequent pages that could cause a single-row UPDATE to take hours So the DBMS must find some free space in another page and put the new data in that page In theory there are different ways to do this, but we found that DBMSs prefer these choices:
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The row that gets moved is the row that's being updated not the row that would otherwise be shifted past the end of the page This choice saves time because if the DBMS moved a different row, it would have to get a lock on it before the UPDATE could proceed The new row location will probably be close to the old one, but it's not possible to guarantee that Most DBMSs will either look for space within the same extent or will search through a list of pages that have free space in them (Oracle calls this list the freelist) However, if all pages are full, the DBMS has to allocate a new extent and put the changed row at the end The DBMS puts in a pointer at the original row location The pointer is the row identifier of the new location So even if the new row location is Page #7 Row #7, you can still access it at Page #1 Row #1 (the DBMS will automatically follow the pointer) That means if you accessed the row using a row identifier, you are safe because the original ROWID is still valid There is also no need to change index keys because they can continue to point to the original location However, all subsequent accesses of this row will take twice as long, until the table is reorganized
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The whole process of finding a home for an expanding UPDATE is called migration As well as causing slow access due to pointers, migration causes space to be wasted That's because the row is gone from the original page All that's left is a pointer, so there's more free space now in the original page
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