IBM Informix Interview Questions and Answers

A chunk is a physical portion of disk on which Informix stores its data. A chunk can be either a raw partition or a file system file. Informix suggests that a chunk’s name be a symbolic link to the actual chunk. For example, if you are using /dev/rdsk/c6t0d0s1 as a chunk, you could create a symbolic link named /dev/informix/chunk1 that points to /dev/rdsk/c6t0d0s1. That way, if there are ever any problems with that disk, you could replace the disk with another one and just change the symbolic link.

from command line# onmonitor ->Logical-Logs->Databases.

Informix IDS provide following modes:

Online mode – This is the normal operating mode of the database server.

Offline mode – The database server is not running. Shared memory is not allocated.

Quiescent mode – This is the normal operating mode of the database server Administrators use this mode to perform maintenance functions that do not require the execution of SQL and DDL statements. Only the administrator (user Informix) can access the database server.

Single-User mode – This mode is an intermediary mode between Quiescent mode and Online mode. Administrators use this mode to perform any maintenance task, including tasks requiring the execution of SQL and DDL statements. Administrators can also perform all other functions available in Online mode. You can see the current database server mode by executing the onstat – utility from the command.

IBM releasing.

The database server uses a concept called key-value locking to lock the deleted row. When the database server deletes a row, key values in the indexes for the table are not removed immediately. Instead, each key value is marked as deleted, and a lock is placed on the key value. One of the most important uses for key-value locking is to assure that a unique key remains unique through the end of the transaction that deleted it.

Without this protection mechanism, user A might delete a unique key within a transaction, and user B might insert a row with the same key before the transaction commits. This scenario makes rollback by user A impossible. Key-value locking prevents user B from inserting the row until the end of user A’s transaction.

A binary large object, also known as a blob, is a collection of binary data stored as a single entity in a database management system. Informix invented an alternative backronym, Binary Large Object. Blobs are typically images, audio or other multimedia objects, though sometimes binary executable code is stored as a blob.

The Informix database management system was conceived and designed by Roger Sippl in the late 1970s. Informix was founded in 1980, went public in 1986, and for a period during the 1990s Informix was the second most popular database system, after Oracle.

An Informix instance is a single connection to shared memory. It is represented by one or more on init processes. When Informix is started or stopped, these processes appear and disappear. Informix used to use the term “server” when referring to an instance, hence the environment variable DBSERVERNAME.

Light scans occur under the following conditions: The optimizer chooses a sequential scan of the table. The number of pages in the table is greater than the number of buffers in the buffer pool. The isolation level obtains no lock or a shared lock on the table: Dirty Read (including non-logging databases) isolation level Repeatable Read isolation level if the table has a shared or exclusive lock

Committed Read isolation if the table has a shared lock.

If a table is designed without specifying extent sizes, Informix uses a default of 16 pages for the first and 8 pages for each subsequent extent. Extent sizing is done when the table is initially created.

By setting PDQ PRIORITY the DBA can change the query execution priority and increase the number of internal threads spawned by Informix to satisfy the query. Also, some memory allocation strategies differ upon setting PDQPRIORITY.

INNER JOIN, LEFT / RIGHT OUTER JOIN, FULL OUTER JOIN, CROSS JOIN

Fragment elimination appears when the following conditions are all satisfied:

• A table is fragmented
• The fragmentation formula doesn’t create overlapping? asta oricum nu e permis (overlapping)
• A query uses equality filter on a column used in the fragmentation formula
• If the above conditions are met, the Informix engine can skip fragments (data is not searched there) that don’t comply with the query filter

• Any object for whom its binary data file can be stored and restored with success into a database can be considered a BLOB.
• As an example, an ecommerce site can store the product photos and videos inside a database. But that’s not a best practice.