How do I run a Db2 Explain Plan?
Perform the following steps to generate the query plan:
- Log into DB2 server as instance owner.
- Ensure that tables exist or just recreate them. Details can be found in DB2 infocenter.
- Save the problematic SQL statement in a file, for example, sql1.sql .
- Use the db2exfmt command to generate a query plan:
How do I run db2advis?
For dynamic SQL statements, the frequency with which statements are executed can be obtained from the monitor as follows:
- Issue the command: db2 reset monitor for database database-alias. Wait for an appropriate interval of time.
- Issue the command: db2advis -g other-options.
What is access path in Db2?
The access path for an SQL statement specifies how Db2 accesses the data that the query specifies. It specifies the indexes and tables that are accessed, the access methods that are used, and the order in which objects are accessed.
What are explain tables?
EXPLAIN tables contain information about SQL statements and functions that run on Db2 for z/OS®. You can create and maintain a set of EXPLAIN tables to capture and analyze information about the performance of SQL statements and functions that run on Db2 for z/OS.
What are explain tables in Db2?
What is the use of explain in Db2?
EXPLAIN indicates whether Db2 used an index to access data, whether sorts were performed, whether parallel processing was used, and so on. As you gain experience working with Db2, you can use the plan table to give optimization hints to Db2 that influence access path selection.
What are different types of locks in DB2?
The modes S, U, and X of table, partition, and table space locks are sometimes called gross lock modes. In the context of reading, SIX is a gross mode lock because you do not get page or row locks; in this sense, it is like an S lock.
What is reorg in DB2?
The DB2 reorg command reorganizes the table and its indexes by rebuilding the index data into unfragmented and contiguous area on the disk. This operating requires locking the data for movement. If there are applications that access this data, then the application performance might be degraded.
What is DB2 plan table?
More Detail. The PLAN_TABLE in DB2 contains details of the access paths used by the optimizer from the EXPLAIN statement. This table gives much information about the PLAN having DB2 SQL statements. ACCESSTYPE is one of the columns of PLAN_TABLE which gives details regarding the access type.
What is the use of explain in DB2?
How do you explain a query?
A query is a request for data or information from a database table or combination of tables. This data may be generated as results returned by Structured Query Language (SQL) or as pictorials, graphs or complex results, e.g., trend analyses from data-mining tools.
Which method is used in a DB2 execute query?
executeQuery method. To retrieve data from a table using a SELECT statement with parameter markers, you use the PreparedStatement.
What are different types of locks in Db2?
What is the difference between exclusive lock and shared lock?
The two types are exclusive and shared locks. Exclusive locks can be active or retained; shared locks can only be active (see Active and retained states for locks ). Note that there are no delete locks in RLS mode.
What is the difference between update lock and exclusive lock?
Update Lock is kind of Exclusive Lock except it can be placed on the row which already have Shared Lock on it. Update Lock reads the data of row which has Shared Lock, as soon as Update Lock is ready to change the data it converts itself to Exclusive Lock.
Why reorg is required?
The reorgchk and reorg commands can improve both search and update operation performance. The DB2 reorg command reorganizes the table and its indexes by rebuilding the index data into unfragmented and contiguous area on the disk. This operating requires locking the data for movement.
Why do we use Runstats in DB2?
RUNSTATS is a DB2 utility that scans a table space or indexes to gather information about space utilization and index efficiency. The information gathered is stored in the DB2 system tables and used by the SQL optimizer to select the best access paths during the bind process.
What is DMS in DB2?
In a DMS (database managed space) table space, the database manager controls the storage space. Unlike SMS table spaces, storage space is pre-allocated on the file system based on container definitions that you specify when you create the DMS table space.
What are the types of queries?
A few of the more popular queries include:
- Single-Table Select query.
- Multiple-Table Select query.
- Range query.
- Complex query.
- Totals query.
- Action query.
- Parameter query.
- Crosstab query.
How do you optimize a query?
It’s vital you optimize your queries for minimum impact on database performance.
- Define business requirements first.
- SELECT fields instead of using SELECT *
- Avoid SELECT DISTINCT.
- Create joins with INNER JOIN (not WHERE)
- Use WHERE instead of HAVING to define filters.
- Use wildcards at the end of a phrase only.
What does Runstats do in Db2?
What are isolation levels in Db2?
Db2 implements page and row locking at the program execution level, which means that all page or row locks are acquired as needed during the program run.
…
In Db2 you can specify the following four isolation levels:
- cursor stability (CS),
- repeatable read (RR),
- read stability (RS), and.
- uncommitted read (UR).
Can you write with a shared lock?
A shared or read lock prohibits any other process from requesting a write lock on the specified part of the file.
What are the different modes of lock?
At the table level, there are five different types of locks:
- Exclusive (X)
- Shared (S)
- Intent exclusive (IX)
- Intent shared (IS)
- Shared with intent exclusive (SIX)
What is the difference between shared mode and exclusive lock?
The two types are exclusive and shared locks. Exclusive locks can be active or retained; shared locks can only be active (see Active and retained states for locks ).