vb.net barcode component Daft Data Types in Java

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One of the first examples in this chapter examined the predicate date_col between 30th Dec 2002 and 5th Jan 2003 on a table holding data from 1 January 2000 to 31 December 2004, and we saw how the optimizer could work out the selectivity of such a predicate. But let s see how things can go wrong in popular implementations of storing dates. We create and populate a table with one row per date over a five-year date range (see script date_oddity.sql in the online code suite).
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CHAPTER 6 SELECTIVITY ISSUES
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create table t1 ( d1 n1 v1 ) ;
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date, number(8), varchar2(8)
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insert into t1 select d1, to_number(to_char(d1,'yyyymmdd')), to_char(d1,'yyyymmdd') from ( select to_date('31-Dec-1999') + rownum from all_objects where rownum <= 1827 ) ;
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In this example, I have stored the same information in three different ways. The first column is a proper Oracle date. The second and third columns are popular options for the database-independent applications that tend to store the date as a character string in the format YYYYMMDD (year/month/day) or its numeric equivalent. How does this affect the way the optimizer copes with the following simple queries select from where ; select from where ; select from where ; * t1 n1 between 20021230 and 20030105 * t1 d1 between to_date('30-Dec-2002','dd-mon-yyyy') and to_date('05-Jan-2003','dd-mon-yyyy')
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* t1 v1 between '20021230' and '20030105'
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CHAPTER 6 SELECTIVITY ISSUES
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In all three cases, Oracle will have to do a full tablescan as we haven t created any indexes but how many rows does the optimizer think the query will return Run each query through autotrace, and check the cardinality. Table 6-1 shows the results for a few different versions of Oracle.
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Table 6-1. Different Data Types Give Different Cardinality
Column Type
Date Numeric Character
Cardinality 8.1.7.4
9 397 457
Cardinality 9.2.0.4/10.1.0.2
8 396 457
Cardinality 9.2.0.6 /10.1.0.4
8 396 396
The proper date column has produced the right answer (nearly); the error is the usual one of the optimizer using arithmetic appropriate to continuously varying values when we are really handling a relatively small list of discrete values. Oracle understands what dates are, what they mean, and how to do arithmetic with them. But what s gone wrong with the numeric and character versions Nothing, really we ve just hidden from the optimizer the fact that they are really holding dates, so the optimizer has used the standard arithmetic on a data set that is a little peculiar. Remember the standard formula for a range in this case using the version that is closed at both ends (between is a short-hand for '>= X and <= Y'). Let s apply it for the numeric case: Selectivity = (required range) / (high value - low value) + 2/num_distinct = (20030105 - 20021230) / (20041231 - 20000101) + 2/1827 = 8875 / 41130 + 2/1827 = 0.215779 + 0.001095 = 0.216874 -- more than 20% of the data apparently! To finish things off, multiply the selectivity by the number of rows in the table (1,827) and we get 1,827 * 0.216874 = 396.228. The optimizer doesn t know that we are dealing with dates, so although we see numbers that we can recognize as a date range that crosses a month boundary or a year boundary, the optimizer doesn t see this as the next day, it simply sees a huge gap and we haven t given the optimizer any information about gaps. It s inevitable that the arithmetic will go wrong. To make it easier to show that the value of 457 reported for the character types in older versions is following the standard formulae, I created a PL/SQL function to apply the conversion algorithm I described earlier on in the chapter (see script char_fun.sql in the online code suite). Consequently, I can just run the following SQL statement to work out the cardinality:
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