[bbk2344] 第49集 - Chapter 12-Optimizing Sore Perations(04)

Generating Histograms

Histogram statistics are generated by:

SQL>EXECUTE dbms_stats.gather_table_stats('HR','EMPLOYEES',METHOD_OPT => 'FOR COLUMNS SIZE 10 salary') ;

 

Histograms are specified using the METHOD_OPT argument of the DBMS_STATS gathering procedure.Oracle recommends setting the MEHTHOD_OPT to FOR ALL COLUMNS SIZE AUTO.Wtih this setting,Oracle automatically determines which columns require histograms and the number of buckets(size) of each histogram.You can also manually specify which columns should have histograms and the size of each histogram.

SQL> begin
  2  for i in 1..100
  3  loop
  4      insert into t values(i,'arcerzhang');
  5  end loop;
  6  commit;
  7  end;
  8  /

PL/SQL procedure successfully completed.

SQL> select * from t;

        ID NAME
---------- ------------------------------
         1 arcerzhang
         2 arcerzhang
         3 arcerzhang
         4 arcerzhang
         5 arcerzhang
         6 arcerzhang
         7 arcerzhang
         8 arcerzhang
         9 arcerzhang
        10 arcerzhang
        11 arcerzhang

        ID NAME
---------- ------------------------------
        12 arcerzhang
        13 arcerzhang
        14 arcerzhang
        15 arcerzhang
        16 arcerzhang
        17 arcerzhang
        18 arcerzhang
        19 arcerzhang
        20 arcerzhang
        21 arcerzhang
        22 arcerzhang

        ID NAME
---------- ------------------------------
        23 arcerzhang
        24 arcerzhang
        25 arcerzhang
        26 arcerzhang
        27 arcerzhang
        28 arcerzhang
        29 arcerzhang
        30 arcerzhang
        31 arcerzhang
        32 arcerzhang
        33 arcerzhang

        ID NAME
---------- ------------------------------
        34 arcerzhang
        35 arcerzhang
        36 arcerzhang
        37 arcerzhang
        38 arcerzhang
        39 arcerzhang
        40 arcerzhang
        41 arcerzhang
        42 arcerzhang
        43 arcerzhang
        44 arcerzhang

        ID NAME
---------- ------------------------------
        45 arcerzhang
        46 arcerzhang
        47 arcerzhang
        48 arcerzhang
        49 arcerzhang
        50 arcerzhang
        51 arcerzhang
        52 arcerzhang
        53 arcerzhang
        54 arcerzhang
        55 arcerzhang

        ID NAME
---------- ------------------------------
        56 arcerzhang
        57 arcerzhang
        58 arcerzhang
        59 arcerzhang
        60 arcerzhang
        61 arcerzhang
        62 arcerzhang
        63 arcerzhang
        64 arcerzhang
        65 arcerzhang
        66 arcerzhang

        ID NAME
---------- ------------------------------
        67 arcerzhang
        68 arcerzhang
        69 arcerzhang
        70 arcerzhang
        71 arcerzhang
        72 arcerzhang
        73 arcerzhang
        74 arcerzhang
        75 arcerzhang
        76 arcerzhang
        77 arcerzhang

        ID NAME
---------- ------------------------------
        78 arcerzhang
        79 arcerzhang
        80 arcerzhang
        81 arcerzhang
        82 arcerzhang
        83 arcerzhang
        84 arcerzhang
        85 arcerzhang
        86 arcerzhang
        87 arcerzhang
        88 arcerzhang

        ID NAME
---------- ------------------------------
        89 arcerzhang
        90 arcerzhang
        91 arcerzhang
        92 arcerzhang
        93 arcerzhang
        94 arcerzhang
        95 arcerzhang
        96 arcerzhang
        97 arcerzhang
        98 arcerzhang
        99 arcerzhang

        ID NAME
---------- ------------------------------
       100 arcerzhang

100 rows selected.

SQL> EXEC dbms_stats.gather_table_stats('HR','T',method_opt=>'FOR COLUMNS SIZE 10 id');

PL/SQL procedure successfully completed.

 

SQL> SELECT endpoint_number,endpoint_value FROM dba_histograms WHERE owner='HR' and table_name = 'T' AND column_name = 'ID';

ENDPOINT_NUMBER ENDPOINT_VALUE
--------------- --------------
              0              1
              1             10
              2             20
              3             30
              4             40
              5             50
              6             60
              7             70
              8             80
              9             90
             10            100

11 rows selected.

 

Gather Statistics Estimates

• dbms_stats.auto_samp[le_size:
• New estimate_percent value
• MEHTOD_OPT options:
  • -REPEAT:New histogram with same number of buckets
  • -AUTO:Net histogram based on data distribution and application workload
  • -SKEWONLY:New histogram based on data distribution

SQL>EXECUTE dbms_status.gather_schema_stats(ownname => 'OE',estimate_percent => DBMS_STATS.AUTO_SAMPLE_SIZE,method_opt => 'for all columns size AUTO');

 

Auto Statistics Collecting

For the dbms_status.gather_schema_stats procedure set OPTIONS to:

• Gather stale
• Gather Empty
• Gather Auto

SQL>EXEC dbms_stats.gather_schema_stats(OWNNAME => 'OE',OPTIONS => 'GATHER AUTO');

 

如何让一张表处于被监控状态?

alter table hr.t monitoring; 

Statistics分为四大类

• Table
• Index
• Column
• System

Optimizer Cost Model

• Three columns in plan_table are:
  • -cpu_cost:Estimated CPU cost of the operation
  • -io_cost:Estimated I/O cost of the operation
  • -temp_space:Estimated temporary space(in bytes)
• Include CPU usage
• Accounts for the effect of caching
• Accounts for index prefetching