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CHAPTER 3 SINGLE TABLE SELECTIVITY
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Once we know how to deal with the simple case of column = constant, we can move on to slightly more complex cases, such as queries involving lists, queries involving lists with nulls, queries involving two columns, queries involving ranges, and queries involving bind variables. There are plenty of cases to investigate before we worry about indexes and joins. Let s start with the easiest option, in-lists. Sticking with the table that represents our audience of 1,200 people, we can write a query like the following: select from where ; count(*) audience month_no in (6,7,8)
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Given that we ve selected three months, and we expect 100 people per month, we shouldn t be too surprised if the calculated cardinality came out as 300. But I m going to start my investigation with 8i, and unfortunately, the following execution plan is what we get when we build the test with that version of Oracle (see in_list.sql in the online suite): Execution Plan (8.1.7.4) ---------------------------------------------------------0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=1 Card=1 Bytes=3) 1 0 SORT (AGGREGATE) 2 1 TABLE ACCESS (FULL) OF 'AUDIENCE' (Cost=1 Card=276 Bytes=828) Where did that 276 in the full scan come from (Using 9i, or 10g, the cardinality is reported at 300, as expected.) There are two points to consider here. First, the calculation is obviously wrong. Secondly, and more importantly, the calculation changes as you upgrade your version of Oracle and as I pointed out at the start of this chapter, the correct cardinality is crucial to getting the correct join order and optimum choice of indexes. You may see some of your execution plans change for no reason when you upgrade.
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Internally, the optimizer will convert a predicate like month_no in (6,7,8) to month_no = 6 or month_no = 7 or month_no = 8. If you add the use_concat hint to the query, the optimizer will then transform the plan into a union all of its three component parts at which point 8i suddenly produces the correct cardinality. The error that 8i suffers from is that after splitting the list into three separate predicates, it applies the standard algorithm for multiple disjuncts (the technical term for OR ed predicates). This algorithm generically corrects for rows double-counted where the predicates overlap but of course, the predicates generated from an in-list are guaranteed not to overlap. 8i transforms the SQL to produce a very special case of disjuncts, and then fails to process it correctly.
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CHAPTER 3 SINGLE TABLE SELECTIVITY
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When an oddity like this shows up, you need to think of two things. First: is the example a special case (Our column has only 12 distinct values would the problem occur, or be so apparent, if there were more distinct values ) Second: are there closely related areas that might produce other oddities Let s spend a little time experimenting around these two questions to see what happens. The code to create the base table was as follows: create table audience as select trunc(dbms_random.value(1,13)) from all_objects where rownum <= 1200 ;
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Let s change this to generate 1,000 possible months across 12,000 people (so that the optimizer should calculate 12 rows per month number ). create table audience as select trunc(dbms_random.value(1,1001)) from all_objects where rownum <= 12000 ;
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month_no
We can then create a simple script that queries the tables with longer and longer in-lists (the online code suite holds separate scripts for the two data sets: in_list.sql uses the original 1,200 rows and in_list_02.sql uses 12,000 rows with 1,000 distinct values): select select select . . . select count(*) from audience where month_no in (1,2); count(*) from audience where month_no in (1,2,3); count(*) from audience where month_no in (1,2,3,4); count(*) from audience where month_no in (1,2,3,4,5,6,7,8,9,10,11,12,13,14);
select count(*) from audience where month_no in ( 1, 2, 3, 4, 5, 6, 7, 8, 9,10, 11,12,13,14,15,16,17,18,19,20, 21,22,23,24,25,26,27,28,29,30 ); When we produce a table of results, as shown in Table 3-1, you can see that the extreme divergence that appears in the base case where we had only 12 values is much less obtrusive in the example with 1,000 different values. In fact, we see no divergence at all in the large example until the in-list has 14 items in it. Moreover, when you check the values for 9i and 10g, you see that the cardinalities always come out as N * number of entries in the list until the number of entries in the list exceeds the number of distinct values.
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