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If a static field or a static initonly field is initialized within the field s declaration, the C++/CLI compiler automatically emits a static constructor that initializes the field. The following class demonstrates this: ref class TaxRates { static float GetConfigValue(String^ key, float defaultValue) { float value = 0.0f; if (!float::TryParse(ConfigurationManager::AppSettings[key], value)) value = defaultValue; return value; } public: static initonly float GermanVAT = GetConfigValue("GermanVAT", 0.16); static initonly float UKVAT = GetConfigValue("UKVAT", 0.175); }; The CLR is implemented so that static constructors are called in a thread-safe way. For example, if one thread tries to access a static field of a managed type while another thread is currently executing the managed type s initializer, the first thread has to wait. In rare cases, you may want to ensure that a type initializer is called even though you are not about to use a type directly. You can achieve this by calling System::Runtime::CompilerServices::RuntimeHelpers::RunClassConstructor, passing the type info object for the type you want to preinitialize.

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One of the trickiest and most satisfying aspects of a DBA s job is helping to improve the quality of SQL code in the application. Efficient code means fast performance, and an easy way to decrease the I/O your query requires is to try to lower the number of rows that the Optimizer has to examine. The Optimizer is supposed to find the optimal plan based on your query. This means the Optimizer won t rewrite an inefficiently written query it only produces the execution plan for that query. Also, even if your query is efficiently written, the Optimizer may not always end up producing the best execution plan. You have better knowledge of your application and data than the Optimizer does, and you can, with hints, force the Optimizer to use that knowledge. The following sections cover some of the best guidelines for writing good SQL.

Selective criteria in your WHERE clauses can dramatically decrease the amount of data Oracle has to consider during a query You can follow some simple principles to ensure that the structure of your SQL statements is not inherently inefficient Your join methods may be fine, but overlooking some of these principles could doom your statement from a performance point of view Careful specification of WHERE conditions can have a significant bearing on whether the Optimizer will choose existing indexes The principle of selectivity the number of rows returned by a query as a percentage of the total number of rows in a table is the key idea here A low percentage means high selectivity and a high percentage means the reverse Because more selective WHERE clauses mean fewer I/Os, the CBO tends to prefer to choose those kinds of WHERE clauses over others in the same query.

The following example makes this clear: SQL> SELECT * FROM national_employees WHERE ss_no = 515086789 AND city='DALLAS'; Two WHERE clauses are in this example, but you can see that the first WHERE clause that uses ss_no requires fewer I/Os The column ss_no is the primary key and is highly selective only one row with that ss_no is in the entire table The Optimizer determines the selectivity of each of the two columns in the query by looking at the index statistics, which tell it how many rows in the table contain each of the two column values in the query If neither of the columns has an index, Oracle will use a full table scan to retrieve the answer to the query If both of them have indexes, it will use the more selective (and hence more efficient) index on the ss_no column.

If you think that the Optimizer should have used an index instead of doing a full table scan, then perform the following steps: 1 Views in a query sometimes prevent the use of indexes Check to make sure that the execution plan shows that the correct indexes are being used 2 If you think heavy data skew is in the table, use histograms to provide Oracle with a more accurate representation of the data distribution in the table The CBO assumes a uniform distribution of column data The CBO may forego the use of an index even when a column value is selective, because the column itself is unselective in nature Histograms help by providing the CBO with an accurate picture of the column data distribution I discuss histograms later in this chapter, in the section Using Histograms 3.