| Oracle® Database Data Cartridge Developer's Guide 11g Release 2 (11.2) Part Number E10765-02 |
|
|
PDF · Mobi · ePub |
| Oracle® Database Data Cartridge Developer's Guide 11g Release 2 (11.2) Part Number E10765-02 |
|
|
PDF · Mobi · ePub |
This chapter introduces the Oracle Database extensible optimizer, descibes the concepts of optimization, statistics, selectivity, and cost analysis, provides usage examples, and explains predicate ordering and the dependency model of optimizer.
This chapter contains these topics:
Query Optimization is the process of choosing the most efficient way to execute a SQL statement. When the cost-based optimizer was offered for the first time with Oracle7, Oracle supported only standard relational data. The introduction of objects extended the supported data types and functions. The Extensible Indexing feature discussed in Chapter 9, "Defining Operators" introduces user-defined access methods.
See Also:
Oracle Database Concepts for an introduction to optimization
Oracle Database Performance Tuning Guide for information about using hints in SQL statements
Oracle Database PL/SQL Packages and Types Reference for information about DBMS_STATS
The extensible optimizer feature allows authors of user-defined functions and indexes to create statistics collection, selectivity, and cost functions that are used by the optimizer in choosing a query plan. The optimizer cost model is extended to integrate information supplied by the user to assess CPU and the I/O cost, where CPU cost is the number of machine instructions used, and I/O cost is the number of data blocks fetched.
Specifically, you can:
Associate cost functions and default costs with domain indexes (partitioned or non-partitioned), indextypes, packages, and standalone functions. The optimizer can obtain the cost of scanning a single partition of a domain index, multiple domain index partitions, or an entire index.
Associate selectivity functions and default selectivity with methods of object types, package functions, and standalone functions. The optimizer can estimate user-defined selectivity for a single partition, multiple partitions, or the entire table involved in a query.
Associate statistics collection functions with domain indexes and columns of tables. The optimizer can collect user-defined statistics at both the partition level and the object level for a domain index or a table.
Order predicates with functions based on cost.
Select a user-defined access method (domain index) for a table based on access cost.
Use the DBMS_STATS package to invoke user-defined statistics collection and deletion functions.
Use new data dictionary views to include information about the statistics collection, cost, or selectivity functions associated with columns, domain indexes, indextypes or functions.
Add a hint to preserve the order of evaluation for function predicates.
Please note that only the cost-based optimizer has been enhanced; Oracle has not altered the operation of the rule-based optimizer.
The optimizer generates an execution plan for SQL queries and DML statements SELECT, INSERT, UPDATE, or DELETE. For simplicity, we describe the generation of an execution plan in terms of a SELECT statement, but the process for DML statements is similar.
An execution plan includes an access method for each table in the FROM clause, and an ordering, called the join order, of the tables in the FROM clause. System-defined access methods include indexes, hash clusters, and table scans. The optimizer chooses a plan by generating a set of join orders, or permutations, by computing the cost of each, and then by selecting the process with the lowest cost. For each table in the join order, the optimizer computes the cost of each possible access method and join method and chooses the one with the lowest cost. The cost of the join order is the sum of the access method and join method costs. The costs are calculated using algorithms that comprise the cost model. The cost model includes varying level of detail about the physical environment in which the query is executed.
The optimizer uses statistics about the objects referenced in the query to compute the selectivity and costs. The statistics are gathered using the DBMS_STATS package. The selectivity of a predicate is the fraction of rows in a table that is chosen by the predicate, and it is a number between 0 and 1.
The Extensible Indexing feature allows users to define new operators, indextypes, and domain indexes. For user-defined operators and domain indexes, the Extensible Optimizer feature enables you to control the three main components used by the optimizer to select an execution plan statistics, selectivity, and cost. In the following sections, we describe each of these components in greater detail.
Statistics for tables and indexes can be generated by using the DBMS_STATS package. In general, the more accurate the statistics, the better the execution plan generated by the optimizer.
The Extensible Optimizer feature lets you define statistics collection functions for domain indexes, indextypes, data types, individual table columns, and partitions. This means that whenever a domain index is analyzed, a call is made to the user-specified statistics collection function. The database does not know the representation and meaning of the user-collected statistics.
In addition to domain indexes, Oracle supports user-defined statistics collection functions for individual columns of a table, and for user-defined data types. In the former case, whenever a column is analyzed, the user-defined statistics collection function is called to collect statistics in addition to any standard statistics that the database collects. If a statistics collection function exists for a data type, it is called for each column of the table being analyzed that has the required type.
The cost of evaluating a user-defined function depends on the algorithm and the statistical properties of its arguments. It is not practical to store statistics for all possible combinations of columns that could be used as arguments for all functions. Therefore, Oracle maintains only statistics on individual columns. It is also possible that function costs depend on the different statistical properties of each argument. Every column could require statistics for every argument position of every applicable function. Oracle does not support such a proliferation of statistics and cost functions because it would decrease performance.
A user-defined function to drop statistics is required whenever there is a user-defined statistics collection function.
When using system-managed local domain indexes, you must implement two methods of the ODCIStats interface: ODCIStatsExchangePartition(), and ODCIStatsUpdPartStatistics().
The optimizer uses statistics to calculate the selectivity of predicates. The selectivity is the fraction of rows in a table or partition that is chosen by the predicate. It is a number between 0 and 1. The selectivity of a predicate is used to estimate the cost of a particular access method; it is also used to determine the optimal join order. A poor choice of join order by the optimizer could result in a very expensive execution plan.
Currently, the optimizer uses a standard algorithm to estimate the selectivity of selection and join predicates. However, the algorithm does not always work well in cases in which predicates contain functions or type methods. In addition, predicates can contain user-defined operators about which the optimizer does not have any information. In that case the optimizer cannot compute an accurate selectivity.
For greater control over the optimizer's selectivity estimation, this feature lets you specify user-defined selectivity functions for predicates containing user-defined operators, standalone functions, package functions, or type methods. The user-defined selectivity function is called by the optimizer whenever it encounters a predicate with one of the forms shown in Example 10-1:
Example 10-1 Three Predicate Forms that Trigger a Call to the Optimizer
operator(...) relational_operator constant constant relational_operator operator(...) operator(...) LIKE constant
where
operator(...) is a user-defined operator, standalone function, package function, or type method,
relational_operator is one of {<, <=, =, >=, >}, and
constant is a constant value expression or bind variable.
For such cases, users can define selectivity functions associated with operator(...). The arguments to operator can be columns, constants, bind variables, or attribute references. When optimizer encounters such a predicate, it calls the user-defined selectivity function and passes the entire predicate as an argument (including the operator, function, or type method and its arguments, the relational operator relational_operator, and the constant expression or bind variable). The return value of the user-defined selectivity function must be expressed as a percent, and be between 0 and 100 inclusive; the optimizer ignores values outside this range.
Wherever possible, the optimizer uses user-defined selectivity values. However, this is not possible in the following cases:
The user-defined selectivity function returns an invalid value (less than 0 or greater than 100).
There is no user-defined selectivity function defined for the operator, function, or method in the predicate.
The predicate does not have one of the forms listed in Example 10-1; it may also be of the form operator(...) + 3 relational_operator constant.
In each of these cases, the optimizer uses heuristics to estimate the selectivity.
The optimizer estimates the cost of various access paths to choose an optimal plan. For example, it computes the CPU and I/O cost of using an index and a full table scan to choose between the two. However the optimizer does not know the internal storage structure of domain indexes, and so it cannot compute a good estimate of the cost of a domain index.
For greater flexibility, the cost model has been extended to let you define costs for domain indexes, index partitions, and user-defined standalone functions, package functions, and type methods. The user-defined costs can be in the form of default costs that the optimizer looks up, or they can be full-fledged cost functions which the optimizer calls to compute the cost.
Like user-defined selectivity statistics, user-defined cost statistics are optional. If no user-defined cost is available, the optimizer uses heuristics to compute an estimate. However, in the absence of sufficient useful information about the storage structures in user-defined domain indexes and functions, such estimates can be very inaccurate and result in the choice of a sub-optimal execution plan.
User-defined cost functions for domain indexes are called by the optimizer only if a domain index is a valid access path for a user-defined operator (for details regarding when this is true, see the discussion of user-defined indexing in the previous chapter). User-defined cost functions for functions, methods and domain indexes are only called when a predicate has one of the forms outlined in Example 10-1, which is identical to the conditions for user-defined selectivity functions.
User-defined cost functions can return three cost values, each value representing the cost of a single execution of a function or domain index implementation:
CPU — the number of machine cycles executed by the function or domain index implementation. This does not include the overhead of invoking the function.
I/O — the number of data blocks read by the function or domain index implementation. For a domain index, this does not include accesses to the Oracle table. The multiblock I/O factor is not passed to the user-defined cost functions.
NETWORK — the number of data blocks transmitted. This is valid for distributed queries, functions, andand domain index implementations. For Oracle this cost component is not used and is ignored; however, as described in the following sections, the user is required to stipulate a value so that backward compatibility is facilitated when this feature is introduced.
The optimizer computes a composite cost from these cost values.
The package DBMS_ODCI contains a function estimate_cpu_units to help get the CPU and I/O cost from input consisting of the elapsed time of a user function. estimate_cpu_units measures CPU units by multiplying the elapsed time by the processor speed of the machine and returns the approximate number of CPU instructions associated with the user function. For a multiprocessor machine, estimate_cpu_units considers the speed of a single processor.
The cost of a query is a function of the cost values. The settings of optimizer initialization parameters determine which cost to minimize. If optimizer_mode is first_rows, the resource cost of returning a single row is minimized, and the optimizer mode is passed to user-defined cost functions. Otherwise, the resource cost of returning all rows is minimized.
You can compute and store user-defined statistics for domain indexes and columns. User-defined selectivity and cost functions for functions and domain indexes can use both standard and user-defined statistics in their computation. The internal representation of these statistics need not be known to Oracle, but you must provide methods for their collection. You are solely responsible for defining the representation of such statistics and for maintaining them. Note that user-collected statistics are used only by user-defined selectivity and cost functions; the optimizer uses only its standard statistics.
User-defined statistics collection, selectivity, and cost functions must be defined in a user-defined type. Depending on the functionality you want it to support, this type must implement as methods some or all of the functions defined in the system interface ODCIStats, Oracle Data Cartridge Interface Statistics, in Chapter 21, "Extensible Optimizer Interface".
Example 10-2 shows a type definition (or the outline of one) that implements all the functions in the ODCIStats interface.
Example 10-2 Defining a Statistics Type
CREATE TYPE my_statistics AS OBJECT (
-- Function to get current interface
FUNCTION ODCIGetInterfaces(ifclist OUT ODCIObjectList) RETURN NUMBER,
-- User-defined statistics functions
FUNCTION ODCIStatsCollect(col ODCIColInfo, options ODCIStatsOptions,
statistics OUT RAW, env ODCIEnv) RETURN NUMBER,
FUNCTION ODCIStatsCollect(ia ODCIIndexInfo, options ODCIStatsOptions,
statistics OUT RAW, env ODCIEnv) RETURN NUMBER,
FUNCTION ODCIStatsDelete(col ODCIColInfo, statistics OUT RAW, env ODCIEnv)
RETURN NUMBER,
FUNCTION ODCIStatsDelete(ia ODCIIndexInfo, statistics OUT RAW, env ODCIEnv)
RETURN NUMBER,
-- User-defined statistics functions for local domain index
FUNCTION ODCIStatsUpdPartStatistics(ia ODCIIndexInfo, palistODCIPartInfoList,
env ODCIEnv) RETURN NUMBER;
FUNCTION ODCIStatsExchangePartition(ia ODCIIndexInfo, ia1 ODCIIndexInfo,
env ODCIEnv) RETURN NUMBER;
-- User-defined selectivity function
FUNCTION ODCIStatsSelectivity(pred ODCIPredInfo, sel OUT NUMBER, args
ODCIArgDescList, start <function_return_type>,
stop <function_return_type>, <list of function arguments>,
env ODCIEnv) RETURN NUMBER,
-- User-defined cost function for functions and type methods
FUNCTION ODCIStatsFunctionCost(func ODCIFuncInfo, cost OUT ODCICost,
args ODCIArgDescList, <list of function arguments>) RETURN NUMBER,
-- User-defined cost function for domain indexes
FUNCTION ODCIStatsIndexCost(ia ODCIIndexInfo, sel NUMBER,
cost OUT ODCICost, qi ODCIQueryInfo, pred ODCIPredInfo,
args ODCIArgDescList, start <operator_return_type>,
stop <operator_return_type>, <list of operator value arguments>,
env ODCIEnv) RETURN NUMBER
)
The object type that you define, referred to as a statistics type, need not implement all the functions from ODCIStats. User-defined statistics collection, selectivity, and cost functions are optional, so a statistics type may contain only a subset of the functions in ODCIStats. Table 10-1 lists the type methods and default statistics associated with different kinds of schema objects.
Table 10-1 Statistics Methods and Default Statistics for Various Schema Objects
| ASSOCIATE STATISTICS | Statistics Type Methods Used | Default Statistics |
|---|---|---|
|
column |
||
|
object type |
ODCIStatsCollect(), ODCIStatsDelete(), ODCIStatsFunctionCost(), ODCIStatsSelectivity() |
cost, selectivity |
|
function |
cost, selectivity |
|
|
package |
cost, selectivity |
|
|
index |
cost |
|
|
indextype |
ODCIStatsCollect(), ODCIStatsDelete(), ODCIStatsIndexCost(), ODCIStatsUpdPartStatistics(), ODCIStatsExchangePartition() |
cost |
The types of the parameters of statistics type methods are system-defined ODCI data types. These are described in Chapter 21, "Extensible Optimizer Interface".
The selectivity and cost functions must not change any database or package state. Consequently, no SQL DDL or DML operations are permitted in the selectivity and cost functions. If such operations are present, the functions are not called by the optimizer.
There are two user-defined statistics collection functions, one for collecting statistics and the other for deleting them.
The first, ODCIStatsCollect(), is used to collect user-defined statistics; its interface depends on whether a column or domain index is being analyzed. It is called when analyzing a column of a table or a domain index and takes two parameters:
col for the column being analyzed, or ia for the domain index being analyzed;
options for options specified in the DBMS_STATS package.
As mentioned, the database does not interpret statistics collected by ODCIStatsCollect(). For system-managed domain index statistics, you don't return the statistics collected by ODCIStatsCollect(). You should store these statistics in a user-managed format, as described in section "Generating Statistics for System-Managed Domain Indexes", and illustrated in Figure 10-1, Figure 10-2, and Figure 10-3.
User-collected statistics are deleted by calling the ODCIStatsDelete() function whose interface depends on whether the statistics for a column or domain index are being dropped. It takes a single parameter: col, for the column whose user-defined statistics must be deleted, or ia, for the domain index whose statistics are to be deleted.
If a user-defined ODCIStatsCollect() function is present in a statistics type, the corresponding ODCIStatsDelete() function must also be present.
The return values of the ODCIStatsCollect() and ODCIStatsDelete() functions must be Success, Error, or Warning; these return values are defined in a system package ODCIConst.
User-defined selectivity functions are used only for predicate forms listed in Example 10-1.
A user-defined selectivity function ODCIStatsSelectivity() takes five sets of input parameters that describe the predicate:
The pred parameter describes the function operator and the relational operator relational_operator.
The args parameter describes the start and stop values (that is, <constant>) of the function and the actual arguments to the function (operator()).
The start parameter, whose data type is identical to that of the function's return value, describes the start value of the function.
The stop parameter, whose data type is identical to that of the function's return value, describes the stop value of the function.
A list of function arguments whose number, position, and type must match the arguments of the function operator.
The computed selectivity is returned in the output parameter sel as a number between 0 and 100 (inclusive) that represents a percentage. The optimizer ignores numbers less than 0 or greater than 100 as invalid values.
The return value of the ODCIStatsSelectivity() function must be one of Success, Error, or Warning.
As an example, consider a function myFunction, as defined in Example 10-3.
A user-defined selectivity function ODCIStatsSelectivity() is detailed in Chapter 21, "Extensible Optimizer Interface".
If myFunction() is called using literal arguments, such as myFunction(2, 'TEST') > 5, then the selectivity function is called as out lined in Example 10-4.
Example 10-4 Calling a Selectivity Function Using Literal Arguments
ODCIStatsSelectivity(ODCIPredInfo_constructor, sel, ODCIArgDescList_constructor, 5, NULL, 2, 'TEST', ODCIEnv_flag)
If, on the other hand, myFunction() is called with some non-literals arguments, such as myFunction(Test_tab.col_a, 'TEST')> 5, where col_a is a column in table Test_tab, then the selectivity function is called as outlined in Example 10-5.
Example 10-5 Calling a Selectivity Function Using Non-Literal Arguments
ODCIStatsSelectivity(ODCIPredInfo_constructor, sel, ODCIArgDescList_constructor, 5, NULL, NULL, 'TEST', ODCIEnv_flag)
In summary, the start, stop, and function argument values are passed to the selectivity function only if they are literals; otherwise they are NULL. ODCIArgDescList describes all the arguments that follow it.
User-defined cost functions are only used for predicate forms listed in Example 10-1.
You can define a function, ODCIStatsFunctionCost(), for computing the cost of standalone functions, package functions, or type methods. This function takes three sets of input parameters describing the predicate:
The func parameter describes the function operator.
The args parameter describes the actual arguments to the function operator.
A list of function arguments whose number, position, and type must match the arguments of the function operator.
The ODCIStatsFunctionCost() function returns its computed cost in the cost parameter. The returned cost can have two components, a CPU cost and an I/O cost, which are combined by the optimizer to compute a composite cost. The costs returned by user-defined cost functions must be positive whole numbers. Invalid values are ignored by the optimizer.
The return value of the ODCIStatsFunctionCost() function must be one of Success, Error, or Warning.
Consider a myFunction(), defined in Example 10-3.
A user-defined cost function ODCIStatsFunctionCost() is detailed in Chapter 21, "Extensible Optimizer Interface".
If myFunction() is called using literal arguments, such as myFunction(2, 'TEST') > 5, where col_a is a column in table Test_tab, then the cost function is called as out lined in Example 10-6.
Example 10-6 Calling a Cost Function Using Literal Arguments
ODCIStatsFunctionCost(ODCIFuncInfo_constructor, cost, ODCIArgDescList_constructor, 2, 'TEST', ODCIEnv_flag)
If, on the other hand, myFunction() is called with non-literal arguments, such as myFunction(Test_tab.col_a, 'TEST') > 5, where col_a is a column in table Test_tab, then the cost function is called as out lined in Example 10-7.
Example 10-7 Calling a Cost Function Using Non-Literal Arguments
ODCIStatsFunctionCost(ODCIFuncInfo_constructor, cost, ODCIArgDescList_constructor, NULL, 'TEST', ODCIEnv_flag)
In summary, function argument values are passed to the cost function only if they are literals; otherwise, they are NULL. ODCIArgDescList describes all the arguments that follow it.
User-defined cost functions for domain indexes are used for the same type of predicates mentioned previously, except that operator must be a user-defined operator for which a valid domain index access path exists.
The ODCIStatsIndexCost() function takes these sets of parameters:
ia describing the domain index
sel representing the user-computed selectivity of the predicate
cost giving the computed cost
qi containing additional information about the query
pred describing the predicate
args describing the start and stop values (that is, <constant>) of the operator and the actual arguments to the operator operator
start, whose data type is identical to that of the operator's return value, describing the start value of the operator
stop whose data type is identical to that of the operator's return value, describing the stop value of the operator
a list of operator value arguments whose number, position, and type must match the arguments of the operator operator. The value arguments of an operator are the arguments excluding the first argument.
env, an environment flag set by the server to indicate which call is being made in cases where multiple calls are made to the same routine. The flag is reserved for future use; currently it is always set to 0.
The computed cost of the domain index is returned in the output parameter, cost.
ODCIStatsIndexCost() returns Success, Error or Warning.
Consider an operator defined in Example 10-8, which returns 1 or 0 depending on whether or not the string b_string is contained in the string a_string. Further, assume that the operator is implemented by a domain index.
A user-defined index cost function ODCIStatsIndexCost() is detailed in Chapter 21, "Extensible Optimizer Interface".
If contains() is called using non-literal arguments, such as Contains(Test_tab.col_c,'TEST') <= 1, then the index cost function is called as out lined in Example 10-9.
Example 10-9 Calling an Index Cost Function Using Non-Literal Arguments
ODCIStatsIndexCost(ODCIIndexInfo_constructor, sel, cost, ODCIQueryInfo_constructor, ODCIPredInfo_constructor, ODCIArgDescList_constructor, NULL, 1, 'TEST', ODCIEnv_flag)
Note that the first argument, a_string, of Contains does not appear as a parameter of ODCIStatsIndexCost(). This is because the first argument to an operator must be a column for the domain index to be used, and this column information is passed in through the ODCIIndexInfo parameter. Only the operator arguments after the first (the value arguments) must appear as parameters to the ODCIStatsIndexCost() function.
In summary, the start, stop, and operator argument values are passed to the index cost function only if they are literals; otherwise they are NULL. ODCIArgDescList describes all the arguments that follow it.
If you choose the system-managed approach to maintain domain indexes and must associate a statistics type with the domain index or the indextype, then the statistics type must also be managed by the system.
Statistics may be collected when issuing an ODCIStatsCollect() call for a system-managed domain index. For a non-partitioned index, the statistics may be stored with the index storage table, as a separate table, or in a data cartridge metadata table with index name qualified rows.
For local partitioned domain indexes, there are three options for storing statistics. All use the ODCIStatsUpdPartStatistics() method during a partition maintenance operation in the following ways. Please note that in all the following examples, no DDLs are executed inside the ODCIStatsUpdPartStatistics() call, and only DML and query instructions are allowed in the implementation of ODCIStatsUpdPartStatistics().
The system calls the ODCIStatsUpdPartStatistics() method If the statistics are stored with the indexed data in the index storage (system-partitioned) tables, as illustrated in Figure 10-1 . The method can optionally maintain any statistics-related partition metadata, or be a null operation. The server deletes or drops the statistics for the affected partitions along with the index data specific to these partitions.
Figure 10-1 Storing Index-Specific Statistics with Index Tables
If the statistics are stored in separate system-partitioned tables, as illustrated in Figure 10-2, the server tracks the creation of these system partitioned tables of store statistics during an ODCIStatsCollect() call. These tables are maintained by the server in the same manner as for index storage tables.
Figure 10-2 Storing Index-Specific Statistics in a Separate Table
If the statistics are stored in a non-partitioned table as either schema-name, index-name, or partition-name qualified rows, as illustrated in Figure 10-3, then you have to maintain the partition-level statistics with a call to ODCIStatsUpdPartStatistics(). The server does not perform any operation on these tables.
Figure 10-3 Storing Index-Partition Statistics in a Common Table
Statistics types act as interfaces for user-defined functions that influence the choice of an execution plan by the optimizer. However, for the optimizer to be able to use a statistics type, it requires a mechanism to bind the statistics type to a database object such as a column, a standalone function, an object type, an index, an indextype or a package. You cannot associate a statistics type with a partition of a table or a partition of a domain index. The ASSOCIATE STATISTICS command creates this association. The following sections describe this command in more detail.
User-defined statistics functions are relevant for columns that use both standard SQL data types and object types, and for domain indexes. The functions ODCIStatsSelectivity(), ODCIStatsFunctionCost(), and ODCIStatsIndexCost() are not used for user-defined statistics, so statistics types used only to collect user-defined statistics need not implement these functions. The following sections describe how to collect column and index user-defined statistics.
Users could create their own tables. This approach requires that privileges on these tables be administered properly, backup and restoration of these tables be done along with other dictionary tables, and point-in-time recovery considerations be resolved.
Consider a table Test_tab, defined as in Example 10-10, where typ1 is an object type.
Example 10-10 Creating a Table with an Object Type Column
CREATE TABLE Test_tab ( col_a NUMBER, col_b typ1, col_c VARCHAR2(2000) )
Suppose that stat is a statistics type that implements ODCIStatsCollect() and ODCIStatsDelete() functions.User-defined statistics are collected by the DBMS_STATS package for the column col_b if we bind a statistics type with the column, as demonstrated in Example 10-11:
Example 10-11 Associating Statistics with Columns for User-Defined Statistics
ASSOCIATE STATISTICS WITH COLUMNS Test_tab.col_b USING stat
A list of columns can be associated with the statistics type stat. Note that Oracle supports only associations with top-level columns, not attributes of object types; if you wish, the ODCIStatsCollect() function can collect individual attribute statistics by traversing the column.
Another way to collect user-defined statistics is to declare an association with a data type, as in Example 10-12, which declares stat_typ1 as the statistics type for the type typ1. When the table Test_tab is analyzed with this association, user-defined statistics are collected for the column col_b using the ODCIStatsCollect() function of statistics type stat_typ1.
Example 10-12 Associating Statistics with Data Types for User-Defined Statistics
ASSOCIATE STATISTICS WITH TYPES typ1 USING stat_typ1
Individual column associations always have precedence over associations with types. Thus, in the preceding example, if both ASSOCIATE STATISTICS commands are issued, DBMS_STATS would use the statistics type stat (and not stat_typ1) to collect user-defined statistics for column col_b. It is also important to note that standard statistics, if possible, are collected along with user-defined statistics.
User-defined statistics are deleted using the ODCIStatsDelete() function from the same statistics type that was used to collect the statistics.
Associations defined by the ASSOCIATE STATISTICS command are stored in a dictionary table called ASSOCIATION$.
Only user-defined data types can have statistics types associated with them; you cannot declare associations for standard SQL data types.
A domain index has an indextype. A statistics type for a system-managed domain index is defined by associating it only with its indextype. Example 10-13 demonstrates how to create an indextype, an index, and an operator on the table Test_tab from Example 10-10:
Example 10-13 Creating an Indextype, an Index and an Operator for User-Defined Statistics
CREATE INDEXTYPE indtype
FOR userOp(NUMBER)
USING imptype WITH SYSTEM MANAGED STORAGE TABLES;
CREATE INDEX Test_indx ON Test_tab(col_a)
INDEXTYPE IS indtype PARAMETERS('example');
CREATE OPERATOR userOp BINDING (NUMBER) RETURN NUMBER
USING userOp_func;
Here, indtype is the indextype, userOp is a user-defined operator supported by indtype, userOp_func is the functional implementation of userOp, and imptype is the implementation type of the indextype indtype.
A statistics type stat_indtype can be associated with the system-managed indextype, as demonstrated in Example 10-14. When the domain index Test_indx that has an indextype indtype is analyzed, user-defined statistics for the index are collected by calling the ODCIStatsCollect() function of stat_indtype.
Example 10-14 Associating Statistics with System-Managed Indextypes
ASSOCIATE STATISTICS WITH INDEXTYPES indtype USING stat_indtype WITH SYSTEM MANAGED STORAGE TABLES
To drop index statistics, use the ODCIStatsDelete() method which is defined for the same statistics type that defined the earlier ODCIStatsCollect() method.
The optimizer uses selectivity functions to compute the selectivity of predicates in a query. The predicates must have one of the appropriate forms and can contain user-defined operators, standalone functions, package functions, or type methods. The following sections describe selectivity computation for each.
Suppose that the association in Example 10-15 is declared. If the optimizer encounters the userOp(Test_tab.col_a) = 1 predicate, it calls the ODCIStatsSelectivity() function (if present) in the statistics type stat_userOp_func that is associated with the functional implementation of the userOp_func of the userOp operator.
If the association in Example 10-16 is declared for a standalone function myFunction, then the optimizer calls the ODCIStatsSelectivity() function (if present) in the statistics type stat_myFunction for the myFunction(Test_tab.col_a, 'TEST') = 1 predicate.
If the association in Example 10-17 is declared for a package Demo_pack, then the optimizer calls the ODCIStatsSelectivity() function (if present) in the statistics type stat_Demo_pack for the Demo_pack.myDemoPackFunction(Test_tab.col_a, 'TEST') = 1 predicate, where myDemoPackFunction is a function in Demo_pack.
If the association in Example 10-18 is declared for a type Example_typ, then the optimizer calls the ODCIStatsSelectivity() function (if present) in the statistics type stat_Example_typ for the myExampleTypMethod(Test_tab.col_b) = 1 predicate, where myExampleTypMethod is a method in Example_typ.
An alternative to selectivity functions is user-defined default selectivity. The default selectivity is a value between 0 and 100%; the optimizer looks it up instead of calling a selectivity function. Default selectivities can be used for predicates with user-defined operators, standalone functions, package functions, or type methods.
The association in Example 10-19 declares that the myFunction(Test_tab.col_a) = 1 predicate always has a selectivity of 20% (or 0.2), regardless of the parameters of myFunction, the comparison operator =, or the constant 1. The optimizer uses this default selectivity instead of calling a selectivity function.
Example 10-19 Associating Statistics with Default Selectivity
ASSOCIATE STATISTICS WITH FUNCTIONS myFunction DEFAULT SELECTIVITY 20
An association can be declared using either a statistics type or a default selectivity, but not both. Thus, the following statement is illegal:
ASSOCIATE STATISTICS WITH FUNCTIONS myFunction USING stat_myFunction DEFAULT SELECTIVITY 20
Other examples of default selectivity declarations include:
ASSOCIATE STATISTICS WITH PACKAGES Demo_pack DEFAULT SELECTIVITY 20 ASSOCIATE STATISTICS WITH TYPES Example_typ DEFAULT SELECTIVITY 20
The optimizer uses user-defined cost functions to compute the cost of predicates in a query. The predicates must have one of the forms listed earlier and can contain user-defined operators, standalone functions, package functions, or type methods. In addition, user-defined cost functions are also used to compute the cost of domain indexes. The following sections describe cost computation for each.
If the association in Example 10-20 is declared, consider the userOp(Test_tab.col_a) = 1 predicate. If the optimizer evaluates the domain index Test_indx with an indtype indextype that implements userOp, it calls the ODCIStatsIndexCost() method (if present) in the statistics type stat_indtype. If the domain index is not used, however, the optimizer calls the ODCIStatsFunctionCost() (if present) in the statistics type stat_userOp to compute the cost of the functional implementation of the operator userOp.
If the association in Example 10-21 is declared for a standalone function myFunction, then the optimizer calls the ODCIStatsFunctionCost() function (if present) in the statistics type stat_myFunction for the myFunction(Test_tab.col_a, 'TEST') = 1 predicate.
Example 10-21 Associating Statistics with Standalone Functions
ASSOCIATE STATISTICS WITH FUNCTIONS myFunction USING stat_myFunction;
User-defined function costs do not influence the choice of access methods; they are only used for ordering predicates, described in Chapter 21, "Extensible Optimizer Interface".
If the association in Example 10-22 is declared for a package Demo_pack, then the optimizer calls the ODCIStatsFunctionCost() function, if present, in the statistics type stat_Demo_pack for the Demo_pack.myDemoPackFunction(Test_tab.col_a) = 1 predicate, where myDemoPackFunction is a function in Demo_pack.
If the association is declared, as in Example 10-23, for a type Example_typ, then the optimizer calls the ODCIStatsFunctionCost() function, if present, in the statistics type stat_Example_typ for the myExampleTypMethod(Test_tab.col_b) = 1 predicate, where myExampleTypMethod is a method in Example_typ.
Like default selectivity, default costs can be used for predicates with user-defined operators, standalone functions, package functions, or type methods. The command in Example 10-24 declares that using the domain index Test_indx to implement the userOp(Test_tab.col_a) = 1 predicate always has a CPU cost of 100, an I/O cost of 5, and a network cost of 0 (the network cost is ignored in Oracle), regardless of the parameters of userOp, the comparison operator "=", or the constant "1". The optimizer uses this default cost instead of calling the ODCIStatsIndexCost() function.
Example 10-24 Associating Statistics with Default Cost
ASSOCIATE STATISTICS WITH INDEXES Test_indx DEFAULT COST (100, 5, 0);
You can declare an association using either a statistics type or a default cost but not both. Thus, the following statement is illegal:
ASSOCIATE STATISTICS WITH INDEXES Test_indx USING stat_Test_indx DEFAULT COST (100, 5, 0)
The following are some more examples of default cost declarations:
ASSOCIATE STATISTICS WITH FUNCTIONS myFunction DEFAULT COST (100, 5, 0) ASSOCIATE STATISTICS WITH PACKAGES Demo_pack DEFAULT COST (100, 5, 0) ASSOCIATE STATISTICS WITH TYPES Example_typ DEFAULT COST (100, 5, 0) ASSOCIATE STATISTICS WITH INDEXTYPES indtype DEFAULT COST (100, 5, 0)
An association of a statistics type defined for an indextype or object type is inherited by index instances of that indextype and by columns of that object type. An inherited association can be overridden by explicitly defining a different association for an index instance or column, but there may be occasions when you would prefer an index or column not to have any association at all. For example, for a particular query the benefit of a better plan may not outweigh the additional compilation time incurred by invoking the cost or selectivity functions. For cases like this, you can use the ASSOCIATE command to declare a NULL association for a column or index, as in Example 10-25.
Example 10-25 Declaring NULL Statistics Associations for Columns and Indexes
ASSOCIATE STATISTICS WITH COLUMNS columns NULL; ASSOCIATE STATISTICS WITH INDEXES indexes NULL;
If the NULL association is specified, the schema object does not inherit any statistics type from the column type or the indextype. A NULL association also precludes default values.
Partition-level and schema object-level aggregate statistics are affected by DDL operations in the same way as standard statistics. Table 10-2 summarizes the effects.
Table 10-2 Effects of DDL on Partition and Global Statistics
| Operation | Effect on Partition Statistics | Effect on Global Statistics |
|---|---|---|
ADD PARTITION |
None |
No Action |
DROP PARTITION |
Statistics deleted |
Statistics recalculated (if _ |
SPLIT PARTITION |
Statistics deleted |
None |
MERGE PARTITION |
Statistics deleted |
None |
TRUNCATE PARTITION |
Statistics deleted |
None |
EXCHANGE PARTITION |
Statistics deleted |
Statistics recalculated (if _ |
REBUILD PARTITION |
None |
None |
MOVE PARTITION |
None |
None |
RENAME PARTITION |
None |
None |
If an existing partition is exchanged, or dropped with an ALTER TABLE DROP PARTITION statement, and the _minimal_stats_aggregation parameter is set to FALSE, the statistics for that partition are deleted, and the aggregate statistics of the table or index are recalculated.
In the absence of an ORDERED_PREDICATES hint, predicates (except those used for index keys) are evaluated in the order specified by the following rules:
Predicates without any user-defined functions, type methods, or subqueries are evaluated first, in the order specified in the WHERE clause.
Predicates with user-defined functions and type methods which have user-computed costs are evaluated in increasing order of their cost.
Predicates with user-defined functions and type methods that have no user-computed cost are evaluated next, in the order specified in the WHERE clause.
Predicates not specified in the WHERE clause (for example, predicates transitively generated by the optimizer) are evaluated next.
Predicates with subqueries are evaluated last in the order specified in the WHERE clause.
The dependency model reflects the actions that are taken when you issue any of the SQL commands described in Table 10-3.
Table 10-3 Dependency Model for DDLs
| Command | Action |
|---|---|
DROP statistics_type
|
If an association is defined with |
DROP statistics_type FORCE
|
Calls |
DROP object
|
Calls |
ALTER TABLE DROP COLUMN |
If association is present for the column, this calls |
DISASSOCIATE |
If user-defined statistics collected with the |
DISASSOCIATE FORCE |
Deletes the entry in |
|
Delete index statistics using the |
The ODCIStatsDelete() function is invoked; if any errors are raised, statistics deletion fails and an error is reported. |
ASSOCIATE |
If an association or user-defined statistics are present for the associated object, the command fails. |
A statistics type is an ordinary object type. Since an object type must have at least one attribute, so must a statistics type. However, because it is never be accessed or set, this is a dummy attribute.
Oracle's distributed implementation does not support adding functions to the remote capabilities list. All functions referencing remote tables are executed as filters. The placement of the filters occurs outside the optimizer. The cost model reflects this implementation and does not attempt to optimize placement of these predicates.
Since predicates are not shipped to the remote site, you cannot use domain indexes on remote tables. Therefore, the DESCRIBE protocol is unchanged, and remote domain indexes are not visible from the local site.
If you are creating an indextype WITH SYSTEM MANAGED STORAGE TABLES, you should also create its associated statistics type WITH SYSTEM MANAGED STORAGE TABLES. If you are collecting statistics on the local indexed column using system partitioned tables, then the Oracle server maintains the system-partitioned statistics tables for them during partition maintenance operations. You can only use the WITH SYSTEM MANAGED STORAGE TABLES option when an indextype is associated with the statistics type; otherwise the system raises an error.
When using local indexes, it may be useful to maintain both partition-level and aggregate object-level statistics. During partition maintenance operations, the partition level statistics are deleted, while the aggregate object-level statistics are either adjusted to reflect the operation or left "as is" for later recomputation.
The decision to adjust or recompute the aggregate statistics is made based on _minimal_stats_aggregation parameter in the server. If the parameter is FALSE, the aggregate statistics are recomputed. If the parameter is TRUE, the statistics are not recomputed.
The system-managed domain indexing approach supports system-managed statistics that are associated with indextypes; indextype itself should also be system-managed.
The cost of execution of the queries remains the same with the extensible optimizer if the same plan is chosen. If a different plan is chosen, the execution time should be better assuming that the user-defined cost, selectivity, and statistics collection functions are accurate. In light of this, you are strongly encouraged to provide statistics collection, selectivity, and cost functions for user-defined structures because the optimizer defaults can be inaccurate and lead to an expensive execution plan.
|
 Copyright © 1996, 2010, Oracle and/or its affiliates. All rights reserved. Legal Notices |
|