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CHAPTER 9 QUERY TRANSFORMATION
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execute pack1.g_ct := 0; -- execute the query execute dbms_output.put_line('Hash table size: ' || ( 32768 - pack1.g_ct)) and we will get the size of the hash table directly; and the answer is
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8i 9i 10g 256 256 Depends on the function s input, output, and the value of parameter _query_execution_cache_max_size
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You will notice that my package definition includes a function that accepts a character input and returns a character output. I wrote this so that I could check whether the hash table behaved differently when large character strings were involved. In 8i and 9i, there was no change in the number of entries in the hash table as I went through various options of mixing character and numeric inputs and outputs. But when I ran a version of the query that supplied the zero-padded, 32-character string, returning the same string,the change in 10g was dramatic: execute pack1.g_ct := 0; select from count(distinct x) ( select /*+ no_merge */ (select pack1.f_v(v32) from dual) x from t1 )
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; execute dbms_output.put_line('Hash table size: ' || ( 32768 - pack1.g_ct)) COUNT(DISTINCTX) ---------------16384 Hash table size: 16 As you can see, the hash table shrank to 16. By selecting a substr() of the function return, I could increase the size of the hash table by decreasing the size of the substring. By supplying character inputs to the function returning a number, I could decrease the size of the hash table by increasing the size of the input string. It seemed likely that the size of the hash table was controlled by a fixed memory limit, rather than an absolute number of hash entries. Since the size of the hash table dropped to 16 when I used a function returning an unconstrained string which really means varchar2(4,000), I looked for any parameter with a default value of 64, or 65,536, and found the parameter _query_execution_cache_max_size. Sure enough,
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CHAPTER 9 QUERY TRANSFORMATION
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you can increase the size of the hash table up to a limit of 16,384 entries (the size is always a power of two) by increasing this parameter. The upshot of this series of test is that you may find after upgrading to 10g that some queries that include scalar subqueries or filter subqueries run more slowly even though their execution plans do not change because their input or output values are quite long. If this is the case, you have two manual options for improving the performance. Try to reduce the total size of the input and output (concatenating strings, explicit substrings), or change the session setting for the _query_execution_cache_max_size parameter (although, as ever, you should be aware that messing with hidden parameters is something that should not be done without approval from Oracle support, and isn t necessarily going to be a stable solution on the next upgrade).
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I don t have to say much about subquery factoring you ve seen it in action a few times in the book already, and every time you ve seen it, I ve used a hint to make the optimizer do what I want rather than taking a cost-based decision. If we take the query I used to create the table for the first example in this chapter, and remove the hint, it looks like this: with generator as ( select rownum id from all_objects where rownum <= 1000 ) select mod(rownum,6), rownum, rownum, rpad('x',60) from generator v1, generator v2 where rownum <= 20000 ; The main body of the query refers (twice) to an object I have called generator, and the initial section of the query tells us how generator is defined. At run time the optimizer has a choice: it can substitute the defining text inline whenever it sees the name generator, or it can create a temporary table once to hold the results of running the query that defines generator and use that temporary table whenever it sees the name generator. Two hints are available for use with subquery factoring: the materialize hint forces the optimizer to create the temporary table, and the inline hint forces the optimizer to replace the name with its defining text and optimize the resulting query. Here are the two possible execution plans generated by the 10g dbms_xplan utility for the preceding query (see script with_subq_01.sql in the online code suite). To keep the plan short,
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