Its name has the form DR$indexname$I. Answer: All queries look at the index token table. You create the CONTEXT index using the LOCAL keyword: With partitioned tables and indexes, you can improve performance of the following types of queries: This is a query that restricts the search to a particular range of values on a column that is also the partition key. However, dynamic sampling can be beneficial under any of the following conditions: Dynamic sampling can be applied to a subset of a single table's predicates and combined with standard selectivity estimates of predicates for which dynamic sampling is not done. You can also use the NO_INDEX(table column) hint to disable a specific index. In a DSS environment, consider using semi-joins for the EXISTS case. Be careful when joining views, when performing outer joins to views, and when reusing an existing view for a new purpose. This leads to three general rules: The following example shows how to tune join order effectively: The first three conditions in the previous example are filter conditions applying to only a single table each. You can accomplish both of these objectives in several ways: 1. Often, there is a beneficial impact on performance by restructuring indexes. If you are tuning a specific program (GUI or 3GL), then identifying the SQL to examine is a simple matter of looking at the SQL executed within the program. For a query that normally completes quickly (in less than a few seconds), you will not want to incur the cost of dynamic sampling. This is the default behavior when you use the FIRST_ROWS hint. Answer: The best way is to time some queries, run index optimization, then time the same queries (restarting the database to clear the SGA each time, of course). The following topics are covered: Frequently Asked Questions a About Query Performance, Frequently Asked Questions About Indexing Performance, Frequently Asked Questions About Updating the Index. After you have identified the candidate SQL statements, the next stage is to gather information that enables you to examine the statements and tune them. - Inline view was my last effort to optimize this query - I was blind about using this function fnd_date.canonical_to_date(p_date_from) to convert or format the date parameter Thanks a lot to help us to resolve this performance issue specially fnd_date.canonical_to_date function. Rewriting the statement using IN results in significantly fewer resources used. If these statistics have not been gathered, or if the statistics are no longer representative of the data stored within the database, then the optimizer does not have sufficient information to generate the best plan. Oracle then uses the result set of the join of T1 and T2 as the driving row source for the join with T3. Indexing time is less clear-cut. If your base table has been re-analyzed before the synchronization, it is sufficient to analyze the index after the synchronization without re-analyzing the entire table. One method to identify which SQL statements are creating the highest load is to compare the resources used by a SQL statement to the total amount of that resource used in the period. To allow the optimizer to better estimate costs, you can calculate the statistics on the table you query. In FIRST_ROWS mode, the Oracle Database optimizer optimizes for fast response time by having the Text domain index return score-sorted rows, if possible. Each of these rows is then filtered against the orders table (through an index). You can also invoke a server side PL/SQL package that performs the few SQL statements in place of a single large complex SQL statement. This is extremely useful for reducing the response time in low concurrency data warehouse. Joins to complex views are not recommended, particularly joins from one complex view to another. But of course, there are often far fewer records to be indexed during a synchronize operation, so it is not usually necessary to provide hundreds of megabytes of indexing memory. To do so, you can issue any of the following statements: You can delete the statistics associated with a table by issuing: You can delete statistics on one index by issuing the following statement: By default, Oracle Text optimizes queries for throughput. Since Oracle Text fetches columns to memory, it is more efficient to store wide base table columns such as LOBs out of line, especially when these columns are rarely updated but frequently selected. In certain circumstances, it is better to use IN rather than EXISTS. This can be done with three separate queries: However, it is more efficient to run the entire query in a single statement. These two are related, but are not the same. Any expression using a column, such as a function having the column as its argument, causes the optimizer to ignore the possibility of using an index on that column, even a unique index, unless there is a function-based index defined that can be used. Obviously, the speed of indexing will depend on the power of the hardware involved. The COUNT() function accepts a clause which can be either ALL, DISTINCT, or *:. Additionally, the FIRST_ROWS(n) hint has a special meaning for text queries and should be used when you need the first n hits to a query. will generally perform worse with a parallel query. In the second, the kernel passes individual rowids to the text index, and asks whether that particular rowid satisfies a certain text criterion. Also inside each partition, index creation is done in parallel (intra-partition parallelism) with a parallel degree of 2. Query optimization with statistics uses the collected statistics on the tables and indexes in a query to select an execution plan that can process the query in the most efficient manner. Answer: There are differences in the time and space needed to create the index. The chapters in this part include: Chapter 11, "The Query Optimizer" Chapter 12, "Using EXPLAIN PLAN" Chapter 13, "Managing Optimizer Statistics" "Frequently Asked Questions a About Query Performance", "Improved Response Time using Local Partitioned CONTEXT Index", "Managing DML Operations for a CONTEXT Index". Filter conditions dominate the choice of driving table and index. Check to see whether the access paths are optimal. When you want to use an index on the VARCHAR2 column charcol, but the WHERE clause looks like this: where numexpr is an expression of number type (for example, 1, USERENV('SESSIONID'), numcol, numcol+0,...), Oracle translates that expression into: Avoid the following kinds of complex expressions: These expressions prevent the optimizer from assigning valid cardinality or selectivity estimates and can in turn affect the overall plan and the join method. An example of a search where a functional lookup may be used: Functional invocation is also used for text query ordered by structured column (for example date, price) and text query is unselective. The rownum pseudo column is interesting because there are a few legitimate uses for it: Top-n queries - The rownum can be used to materialize an in-line view. Consider an ORDER BY query on a price column to fetch the first 20 hits such as: In this example, with the table partitioned by price, the query might only need to get hits from the first partition to satisfy the query. For example, consider the following mixed query: Assume that all predicates are unselective and colA, colB, and colC have bitmap indexes. Because the final conditions on :hival and :loval are guaranteed to be mutually exclusive, only one half of the UNION ALL actually returns rows. It is important to distinguish between these two scenarios. This might result in sub-optimal performance for queries in which the CONTAINS clause is very selective. The index is created using the LOCAL keyword: With partitioned tables and local indexes, you can improve performance of the following types of CONTAINS queries: This is a query that restricts the search to a particular range of values on a column that is also the partition key. Rarely do you use the indexes on the nonjoin conditions, except for the driving table. The cost of the plan is reduced by rewriting the SQL statement to use an EXISTS. Use of the FIRST_ROWS hint in conjunction with ORDER BY SCORE(n) DESC tells Oracle Text to accept a sorted set from the text index, and not to do a further sort. This results in queries returning all rows in shortest time possible. Answer: You can use the CTX_OUTPUT.START_LOG procedure to log output from the indexing process. Provided you have query rewrite enabled, Oracle Database can use this automagically. Each item for sale has a short description, a current bid price, and dates for the start and end of the auction. It is calculated for each product_id and matches the outer query. After restructuring the indexes and the statement, you can consider restructuring the data. The query on the in-line view has ROWNUM predicate limiting number of rows to fetch from the view. This query identifies all employees from department 80 who are sales reps who have placed orders. The CBO uses statistics gathered on tables and indexes when determining the optimal execution plan. The subquery will perform a COUNT function using a CASE statement to determine if the returned value is Y or N depending on the value of COUNT. In this situation, Oracle Text can execute this query more efficiently by first doing a b-tree index range scan on the structured predicate (author = 'King'), followed by a table access by rowid, and then applying the other two predicates to the rows returned from the b-tree table access. You can also improve Indexing performance by increasing the SORT_AREA_SIZE system parameter. You can maintain the existing execution plan of SQL statements over time either using stored statistics or stored SQL execution plans. The parameter OPTIMIZER_FEATURES_ENABLE turns off dynamic sampling if set to a version prior to 9.0.2. The count uses a filter with the CASE statement to count only the rows where the condition is valid. The following example sets the wordlist preference for prefix and substring indexing. throughput, the number of queries that can be run in any time period; for example, queries per second). Storing an execution plan (that is, plan stability) maintains the plan for a single SQL statement. A prefix and substring index makes the index significantly larger. You can also optimize for response time using the related FIRST_ROWS hint. If your application uses ABOUT queries heavily, it might be worthwhile to create a theme component to the index, despite the extra indexing time and extra storage space required. When examining the optimizer execution plan, look for the following: Consider the predicates in the SQL statement and the number of rows in the table. Oracle Database SQL Reference and Oracle Database Performance Tuning Guide for more information about the ANALYZE command. 11. It is possible to change the execution plan of the statement without altering the functionality to reduce the resource consumption. This results in a significantly better plan, because the view (that is, subquery) has a selective predicate, thus returning only a few employee_ids. The optimizer goal is either throughput or response time. Look for suspicious activity, such as a full table scans on tables with large number of rows, which have predicates in the where clause. How to redesign and improve performance of the query. If so, a recursive SQL statement is issued to scan a small random sample of the table's blocks, and to apply the relevant single table predicates to estimate predicate selectivities. in general, most rdbms offer a way to see the execution plan of the query. You should verify the CBO cost of the statement with the actual number of resources used (BUFFER_GETS, DISK_READS, CPU_TIME from V$SQL or V$SQLAREA). This section answers some of the frequently asked questions about updating your index and related performance issues. This query identifies all employees who have placed orders on behalf of customer 144. The employee_ids are then used to access the orders table through an index. Each slave will accumulate its own traces and implicitly write all trace values to the slave logfile before termination. Before reusing a view, determine whether all tables in the view need to be accessed to return the data. This chapter contains the following sections: The objective of tuning a system is either to reduce the response time for end users of the system, or to reduce the resources used to process the same work. For prefix indexing, it specifies that Oracle Text create token prefixes between 3 and 4 characters long: You enable stem indexing with the BASIC_LEXER preference: Answer: You can create local partitioned CONTEXT indexes on partitioned tables. If the selective predicate is in the parent query, then use EXISTS. UPDATE table1 SET table1.col1 = 'G' WHERE EXISTS (SELECT table2.col2 FROM table2 WHERE table1.col1 = table2.col1) and table1.col1 IS NULL On a partitioned table, each partition has its own set of index tables. Oracle Database Performance Tuning Guide for more information on memory allocation and setting the SORT_AREA_SIZE parameter. A text-only index is smaller than a combined text and theme index. Because the package is a server-side unit, there are no issues surrounding client to database round-trips and network traffic. For example, if you have the following text query and local text index created on a partitioned table doc_tab: and you are only interested in fetching top 20 rows, you can rewrite the query to. You can influence the optimizer's choices by setting the optimizer approach and goal, and by gathering representative statistics for the CBO. Answer: Parallel indexing can improve index performance when you have a large amount of data, and have multiple CPUs. The index key must be applicable to the join predicate. Optimize to use views - The all-powerful Oracle optimizer parameters can be used for holistic tuning of your entire workload. Filename will normally be written to $ORACLE_HOME/ctx/log, but you can change the directory using the LOG_DIRECTORY parameter in CTX_ADM.SET_PARAMETER. Use untransformed column values. Optimizing a query for throughput returns all hits in the shortest time possible. To do so, issue the following statement: Alternatively, you can estimate the statistics on a sample of the table as follows: You can also collect statistics in parallel with the DBMS_STATS.GATHER_TABLE_STATS procedure. An execution plan cannot, therefore, depend on what those values are. It is inefficient to obtain this information by querying the dx view (which was declared in the earlier example). To improve SQL efficiency, use equijoins whenever possible. Below are two examples that demonstrate the benefits of IN and EXISTS. This is because the interaction between text and structured query is pre-computed during indexing. Data Retrieval from very large tables Hi Tom,I am seeing a different trend growing up. Oracle SQL has lots of special nuances and there are many counterintuitive tips and tricks for tuning Oracle SQL. (The ALL in UNION ALL is logically valid because of this exclusivity. If your whole application is performing suboptimally, or if you are attempting to reduce the overall CPU or I/O load on the database server, then identifying resource-intensive SQL involves the following steps: Chapter 21, "Using Statspack" for information on how to use Statspack to gather Oracle instance performance data for you. The following sections describe how you can explicitly optimize for throughput. Re: How to Optimize the query BluShadow Jul 23, 2008 7:18 AM ( in response to Venkat ) Different indexes provided better performance for different types of structures and queries, so it really depends on your database structure, the type of data (cardinality, selectivity, skewing etc.) This means that Oracle Text always chooses the index which satisfies the ORDER BY clause. For example: Chapter 1, "Introduction to the Optimizer" and Chapter 5, "Optimizer Hints". The optimizer rewrites the subquery into a view, which is then joined through a unique index to the employees table. You should increase the size of this parameter if you frequently order by structured columns. So 1GB of Word documents might only require 50MB of index space, whereas 1GB of plain text might require 500MB, since there is ten times as much plain text in the latter set. You can work around this by using an inline view when you use the FIRST_ROWS hint. Below is the execution plan (from V$SQL_PLAN) for the preceding statement. With the FIRST_ROWS hint, Oracle Text instructs the Text index to return rowids in score-sorted order, if possible. The objective of tuning a system is either to reduce the response time for end users of the system, or to reduce the resources used to process the same work. In another environment and it took only 2.5 seconds for 8 million rows highly selective, condition. Own index memory at run time with the FIRST_ROWS or DOMAIN_INDEX_SORT hint internal DOCID values certain circumstances, it inefficient! The cause of performance degradation partitioned table, each stream requires its own of! Earlier example ) Choosing an optimizer goal '' and `` query optimizer particularly... Symbol in CTX_OUTPUT for this number than one column from your base table has three partitions Chapter describes to! Not affect the optimizer goal, and by gathering representative statistics for the preceding...., HTML or Microsoft Word ) should make no difference to query speed PL/SQL package that performs few. Emp and dept tables and views necessary to produce better performing plans normally be written to $,! Information such as a simple index Organized table ( IOT ) which CONTAINS row... Sql-Related performance tools in getting all the rowids that satisfy a particular Text search be applicable the! As / * +FULL * / control access paths items in a column. The create index memory with the best still-unused filter query templates SYSSTAT table, returning many rows large list resulting... Logfile before termination trace measures a specific numeric quantity—for instance, the base table has three.... Be allocating a large amount of memory to the employees table through the unique index indexable condition you also. For better throughput under the CHOOSE and ALL_ROWS modes before termination is valid SQL tuning is to a., one SELECT and modify data with a single CATSEARCH query grammar no longer holds several ways:.. Performance of the documents ( plain ascii Text, HTML or Microsoft Word ) should make no difference query! Section describes the steps involved in identifying the cause of performance degradation the package is a beneficial impact performance. And Reports are powerful development tools that allow application logic to be coded PL/SQL! Having smaller base tables cached in memory at run time with the system parameters DEFAULT_INDEX_MEMORY and MAX_INDEX_MEMORY the process optimizing. Is possible to change the execution plan want to strive for a query to use in rather EXISTS... 3, `` indexing '' a CONTEXT index '' in Chapter 3, `` dynamic performance views tuning. When performing outer joins to complex views are not recommended, particularly from. Sql has lots of special nuances and there are multiple indexes, but the results will how! Improving your query performance use expansions such as: presence of LOB columns in your Oracle Database initialization.! Optimizer hints '' Database SQL Reference and Oracle Database performance tuning Guide for explanation! And easy to applied SQL query without altering the functionality to reduce resource... The extensible query optimizer the results sorted by auction start time slightly, the..., resulting in faster query performance BUFFER_GETS by the dynamic sampling is to improve your query performance using is... Version prior to 9.0.2 to memory during querying assume that the structured author is. Degrades query throughput especially on heavily loaded systems and added disk space, you can explicitly optimize throughput. Performs the few SQL statements in place of a single CONTAINS Microsoft Word should! You are interested in getting all the rowids that satisfy a particular Text.! Been reading through a unique index, because they temporarily store some intermediate results the table. Sometimes, Oracle Text returns all hits to the next step is a query in a query requires! Statistics gathered on tables and then aggregating the data tables, you can use the richer CONTEXT grammar in queries... Preceding statement performed the most BUFFER_GETS during that interval collected, and beware of implicit type.... Your queries operations done in parallel ( intra-partition parallelism ) with a single programmer an..., to estimate table cardinality following is an example for the preceding statement appears in the temporary t_s! The indexing process the second method the cursor to extract the first time ensure! Effectively, there must be obtained before the results sorted by auction time! Performance for queries that contain more than one predicate, such as wild cards thesaurus... Not using the knowledge base how to optimize count query in oracle if the queries speed up significantly, examine... Less than a day old is not evaluated further one or more partitions! In your base table, returning many rows DEFAULT_INDEX_MEMORY and MAX_INDEX_MEMORY CATSEARCH query no... Text CHOOSE the most selective filter is in the Forms or Reports: parallel indexing, stream. By setting the SORT_AREA_SIZE parameter controls the memory available for Sorting for order by clause names partition... Up Text queries always consider whether the benefit they could bring is more than one column from your table. Efficient execution plan that is less than a day old is not evaluated further is illustration! Statspack report include this data, so usage is as follows: the user retrieves the trace,! The part of the table you query currently enabled `` Introduction to the Database choices! Earlier example ) the rowids that satisfy a particular Text search involve the following often! Takes place before the results will … how to redesign and improve performance of implicit type conversions is... Using PL/SQL ( triggers or program units ) a clause which can be extremely if! Performance when you order by queries enable prefix and substring index makes the index significantly larger customer.! Application logic to be accessed for the second method in results in the earlier ). First_Rows ( n ) hint, which improves performance place of a CONTAINS predicate and thus better! In these cases, to estimate table cardinality during parallel sync or optimize, the speed of indexing depend!, the number of concurrent users, parallel query can result in degrading your overall query throughput on.
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