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Physical Organization
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How the data is organized physically on disk deeply impacts these calculations, as it materially affects how expensive (or inexpensive) index access will be. Suppose you have a table where the rows have a primary key populated by a sequence. As data is added to the table, rows with sequential sequence numbers might be, in general, next to each other.
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GROUPS will affect how the data is organized on disk. Those features tend to spread the data out, and this natural clustering by primary key may not be observed.
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Note The use of features such as ASSM or multiple FREELIST/FREELIST
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The table is naturally clustered in order by the primary key (since the data is added in more or less that order). It will not be strictly clustered in order by the key, of course (we would have to use an IOT to achieve that); in general, rows with primary keys that are close in value will be close together in physical proximity. When you issue the query select * from T where primary_key between :x and :y the rows you want are typically located on the same blocks. In this case, an index range scan may be useful even if it accesses a large percentage of rows, simply because the database blocks that we need to read and reread will most likely be cached since the data is co-located. On the other hand, if the rows are not co-located, using that same index may be disastrous for performance. A small demonstration will drive this fact home. We ll start with a table that is pretty much ordered by its primary key: ops$tkyte%ORA11GR2> create table colocated ( x int, y varchar2(80) ); Table created. ops$tkyte%ORA11GR2> begin 2 for i in 1 .. 100000 3 loop 4 insert into colocated(x,y) 5 values (i, rpad(dbms_random.random,75,'*') ); 6 end loop; 7 end; 8 / PL/SQL procedure successfully completed. ops$tkyte%ORA11GR2> alter table colocated 2 add constraint colocated_pk 3 primary key(x); Table altered. ops$tkyte%ORA11GR2> begin 2 dbms_stats.gather_table_stats( user, 'COLOCATED'); 3 end; 4 / PL/SQL procedure successfully completed.
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CHAPTER 11 INDEXES
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This table fits the description we laid out earlier with about 100 rows/block in an 8KB database. In this table, there is a very good chance that the rows with X=1, 2, 3 are on the same block. Now, we ll take this table and purposely disorganize it. In the COLOCATED table, we created the Y column with a leading random number, and we ll use that fact to disorganize the data so that it will definitely not be ordered by primary key anymore: ops$tkyte%ORA11GR2> create table disorganized 2 as 3 select x,y 4 from colocated 5 order by y; Table created. ops$tkyte%ORA11GR2> alter table disorganized 2 add constraint disorganized_pk 3 primary key (x); Table altered. ops$tkyte%ORA11GR2> begin 2 dbms_stats.gather_table_stats( user, 'DISORGANIZED'); 3 end; 4 / PL/SQL procedure successfully completed. Arguably, these are the same tables it is a relational database, so physical organization has no bearing on the answers returned (at least that s what they teach in theoretical database courses). In fact, the performance characteristics of these two tables are as different as night and day, while the answers returned are identical. Given the same exact question, using the same exact query plans, and reviewing the TKPROF (SQL trace) output, we see the following: select * from colocated where x between 20000 and 40000 call count ------- -----Parse 5 Execute 5 Fetch 6675 ------- -----total 6685 cpu elapsed disk query current -------- ---------- ---------- ---------- ---------0.00 0.00 0 0 0 0.00 0.00 0 0 0 0.33 0.33 0 14495 0 -------- ---------- ---------- ---------- ---------0.33 0.33 0 14495 0 rows ---------0 0 100005 ---------100005
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Rows Row Source Operation ------- --------------------------------------------------20001 TABLE ACCESS BY INDEX ROWID COLOCATED (cr=2899 pr=0 pw=0 time=8... 20001 INDEX RANGE SCAN COLOCATED_PK (cr=1374 pr=0 pw=0 time=38216 ... ******************************************************************************** select /*+ index( disorganized disorganized_pk ) */* from disorganized where x between 20000 and 40000 call count ------- -----Parse 5 Execute 5 cpu elapsed disk query current -------- ---------- ---------- ---------- ---------0.00 0.00 0 0 0 0.00 0.00 0 0 0 rows ---------0 0
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