Step 4: Choose file organizations and indexes

Transcription

1 Step 4: Choose file organizations and indexes Asst. Prof. Dr. Kanda Saikaew Dept of Computer Engineering Khon Kaen University Overview How to analyze users transactions to determine characteristics that may impact performance. How to select appropriate file organizations based on analysis of transactions. When to select indexes to improve performance 2 Step 4 Choose file organizations and indexes Determine optimal file organizations to store the base tables, and the indexes required to achieve acceptable performance. Consists of the following steps: Step 4.1 Analyze transactions Step 4.2 Choose file organizations Step 4.3 Choose indexes 3 Dr. Kanda Runapongsa Saikaew, Computer Engineering, KKU 1

2 Step 4.1 Analyze transactions To understand functionality of the transactions and to analyze the important ones Identify performance criteria, such as: transactions that run frequently and will have a significant impact on performance transactions that are critical to the business times during the day/week when there will be a high demand made on the database (called the peak load) 4 Step 4.1 Analyze transactions Use this information to identify the parts of the database that may cause performance problems. To select appropriate file organizations and indexes, also need to know highlevel functionality of the transactions, such as: columns that are updated in an update transaction; criteria used to restrict records that are retrieved in a query. 5 Step 4.1 Analyze transactions Often not possible to analyze all expected transactions, so investigate most important ones. To help identify which transactions to investigate, can use: transaction/table cross-reference matrix, showing tables that each transaction accesses, and/or transaction usage map, indicating which tables are potentially heavily used. 6 Dr. Kanda Runapongsa Saikaew, Computer Engineering, KKU 2

3 Step 4.1 Analyze transactions To focus on areas that may be problematic: (1) Map all transaction paths to tables (2) Determine which tables are most frequently accessed by transactions (3) Analyze the data usage of selected transactions that involve these tables 7 Cross-referencing transactions and tables Pearson Education Limited, Transaction usage map for some sample transactions showing expected occurrences 9 Dr. Kanda Runapongsa Saikaew, Computer Engineering, KKU 3

4 Step 4.1 Analyze transactions Data usage analysis For each transaction determine: (a) Tables and columns accessed and type of access. (b) Columns used in any search conditions. (c) For query, columns involved in joins. (d) Expected frequency of transaction. (e) Performance goals of transaction. Pearson Education Limited, Example Transaction Analysis Form Pearson Education Limited, Step 4.2 Choose file organizations To determine an efficient file organization for each base table File organizations include Heap, Hash, Indexed Sequential Access Method (ISAM), B+-Tree, and Clusters. Some DBMSs (particularly PC-based DBMS) have fixed file organization that you cannot alter 12 Dr. Kanda Runapongsa Saikaew, Computer Engineering, KKU 4

5 Data on External Storage Disks: Can retrieve random page at fixed cost But reading several consecutive pages is much cheaper than reading them in random order Tapes: Can only read pages in sequence Cheaper than disks; used for archival storage Data on External Storage File organization: Method of arranging a file of records on external storage. Record id (rid) is sufficient to physically locate record Indexes are data structures that allow us to find the record ids of records with given values in index search key fields Architecture: Buffer manager stages pages from external storage to main memory buffer pool. File and index layers make calls to the buffer manager. Alternative File Organizations Many alternatives exist, each ideal for some situations, and not so good in others: Heap (random order) files: Suitable when typical access is a file scan retrieving all records. Sorted Files: Best if records must be retrieved in some order, or only a `range of records is needed. Dr. Kanda Runapongsa Saikaew, Computer Engineering, KKU 5

6 Alternative File Organizations Indexes: Data structures to organize records via trees or hashing. Like sorted files, they speed up searches for a subset of records, based on values in certain ( search key ) fields Updates are much faster than in sorted files. Indexes (1/2) An index on a file speeds up selections on the search key fields for the index Any subset of the fields of a relation can be the search key for an index on the relation Search key is not the same as key (minimal set of fields that uniquely identify a record in a relation) Indexes (2/2) An index contains a collection of data entries, and supports efficient retrieval of all data entries k* with a given key value k. Given data entry k*, we can find record with key k in at most one disk I/O. (Details soon ) Dr. Kanda Runapongsa Saikaew, Computer Engineering, KKU 6

7 B+ Tree Indexes Non-leaf Pages Leaf Pages (Sorted by search key) Leaf pages contain data entries, and are chained (prev & next) Non-leaf pages have index entries; only used to direct searches: index entry P 0 K 1 P 1 K 2 P 2 K m P m Example B+ Tree Root 17 Note how data entries in leaf level are sorted Entries <= 17 Entries > * 3* 5* 7* 8* 14* 16* 22* 24* 27* 29* 33* 34* 38* 39* Find 28*? 29*? All > 15* and < 30* Insert/delete: Find data entry in leaf, then change it. Need to adjust parent sometimes. And change sometimes bubbles up the tree Hash-Based Indexes (1/2) Good for equality selections. Index is a collection of buckets. Bucket = primary page plus zero or more overflow pages. Buckets contain data entries. Dr. Kanda Runapongsa Saikaew, Computer Engineering, KKU 7

8 Hash-Based Indexes (2/2) Hashing function h: h(r) = bucket in which (data entry for) record r belongs h looks at the search key fields of r. No need for index entries in this scheme. Index Classification Primary vs. secondary: If search key contains primary key, then called primary index. Unique index: Search key contains a candidate key. Clustered vs. unclustered: If order of data records is the same as, or `close to, order of data entries, then called clustered index. Clustered vs. Unclustered Index A file can be clustered on at most one search key Cost of retrieving data records through index varies greatly based on whether index is clustered or not! Secondary indexes provide additional keys for a base table that can be used to retrieve data more efficiently. If ordering column chosen is key of table, index will be a primary index; otherwise, index will be a clustering index. Dr. Kanda Runapongsa Saikaew, Computer Engineering, KKU 8

9 Step 4.3 Choose indexes Determine whether adding indexes will improve the performance of the system Approach 1 Keep records unordered and create as many secondary indexes as necessary 25 Step 4.3 Choose indexes Have to balance overhead in maintenance and use of secondary indexes against performance improvement gained when retrieving data This includes: adding an index record to every secondary index whenever record is inserted updating a secondary index when corresponding record is updated increase in disk space needed to store the secondary index possible performance degradation during query optimization to consider all secondary indexes 26 Step 4.3 Choose indexes Approach 2 Order records in table by specifying a primary or clustering index. In this case, choose the column for ordering or clustering the records as: column that is used most often for join operations - this makes join operation more efficient, or column that is used most often to access the records in a table in order of that column 27 Dr. Kanda Runapongsa Saikaew, Computer Engineering, KKU 9

10 Step 4.3 Choose indexes Guidelines for choosing wish-list (1) Do not index small tables. (2) Index PK of a table if it is not a key of the file organization. (3) Add secondary index to any column that is heavily used as a secondary key. (4) Add secondary index to a FK if it is frequently accessed. (5) Add secondary index on columns that are involved in selection or join criteria; where clause; and sorting (such order by, group by, union, distinct) 28 Step 4.3 Choose indexes Guidelines for choosing wish-list (6) Add secondary index on columns involved in built-in functions. (7) Add secondary index on columns that could result in an index-only plan. (8) Avoid indexing an column or table that is frequently updated. (9) Avoid indexing an column if the query will retrieve a significant proportion of the records in the table. (10) Avoid indexing columns that consist of long character strings. 29 Index-Only Plans (1/3) A number of queries can be answered <E.dno> without retrieving any tuples from one <E.dno,E.sal> or more of the Tree index! relations involved if a <E. age,e.sal> suitable index or is available. <E.sal, E.age> Tree index! SELECT E.dno, COUNT(*) FROM Emp E GROUP BY E.dno SELECT E.dno, MIN(E.sal) FROM Emp E GROUP BY E.dno SELECT AVG(E.sal) FROM Emp E WHERE E.age=25 AND E.sal BETWEEN 3000 AND 5000 Dr. Kanda Runapongsa Saikaew, Computer Engineering, KKU 10

11 Index-Only Plans (2/3) Index-only plans are possible if the key is <dno,age> or we have a tree index with key <age,dno> Which is better? What if we consider the second query? SELECT E.dno, COUNT (*) FROM Emp E WHERE E.age=30 GROUP BY E.dno SELECT E.dno, COUNT (*) FROM Emp E WHERE E.age>30 GROUP BY E.dno Index-Only Plans (3/3) Index-only plans can also be found for queries involving more than one table; more on this later. <E.dno> SELECT D.mgr FROM Dept D, Emp E WHERE D.dno=E.dno <E.dno,E.eid> SELECT D.mgr, E.eid FROM Dept D, Emp E WHERE D.dno=E.dno SQL commands related to index To create an index CREATE INDEX <indexname> ON <tablename> (<column>, <column>...); To enforce unique values, add the UNIQUE keyword: CREATE UNIQUE INDEX <indexname> ON <tablename> (<column>, <column>...); To specify sort order, add the keyword ASC or DESC after each column name To remove an index, simply enter: DROP INDEX <indexname>; 33 Dr. Kanda Runapongsa Saikaew, Computer Engineering, KKU 11

12 Summary Step 4: Choose file organizations and indexes Step 4.1 Analyze transactions Step 4.2 Choose file organizations Step 4.3 Choose indexes Index PK and FK Index on columns that are involved in selection or join criteria; where clause; and sorting (such order by, group by, union, distinct) index on columns that could result in an index-only plan 34 References Connolly and Begg, Database Systems: A Practical Approach to Design, Implementation and Management, Pearson, 2004 Ramakrishnan and Gehrke, Database Management Systems, McGraw-Hill Science/Engineering/Math, Dr. Kanda Runapongsa Saikaew, Computer Engineering, KKU 12