Netezza Interview Questions & Answers

Question 1. What Are The Data Warehouse Appliances You Know?

Answer :

• Netezza
• Greenplum
• Vertica
• Teradata
• Dataupia
• Oracle Exadata

Question 2. What Are The Environment Variables That Are Required To Connect To Netezza?

Answer :

The environment variables required are: NZ_HOST, NZ_DATABASE, NZ_USER, NZ_PASSWORD

Question 3. What Are The Different States Of Netezza?

Answer :

• Online: Normal or usual state.
• Stopped: Netezza will shut down after completing current queries, no new queries allowed.
• Offline: Waits for completion of current queries, new queries in queue receives error.
• Paused: Same as above, but no error displayed. Typically caused during Netezza boot up or startup.
• Down: Just plain down, could be due to Netezza server problem or user initiated.

Question 4. What Are The Constraints On A Table Are Enforced?

Answer :

The only constraint netezza supports is Not null. There are no primary key and foreign key constraints in netezza.

Question 5. Can You Insert Duplicate Rows In Netezza Table?

Answer :

Yes. As there are no primary key constraints in netezza you can insert duplicate rows.

Question 6. How The Not Null Specification On A Column Improves The Netezza Performance?

Answer :

Specifying Not Null on each column in table results in better performance. Netezza tracks the NULL values at rowheader level. Having NULL values results in storing references to NULL values in header. If all columns are NOT NULL, then there is no record header.

Question 7. How Fpga Can Be Helpful In Improving Query Performance?

Answer :

While reading data from the disk, the Field Programmable Gate Array (FPGA) on each SPU filters out unwanted data. This process of data elimination removes IO bottlenecks and frees up downstream components such as the CPU, memory and network from processing extra data.

Question 8. What Is A Snippet?

Answer :

A snippet is a small unit of work that are carried out in SPU.

Question 9. What Are Zonemaps?

Answer :

An extent is the smallest unit of disk allocation on a SPU. Zonemaps is internal mapping structures to the extents that take advantage of the internal ordering of data to eliminate extents that do not need to be scanned. Zonemaps transparently avoid scanning of unreferenced rows. Zonemaps are created for every column in the table and contain the minimum and maximum values for every extent.

Question 10. How The Zonemaps Are Created And Updated?

Answer :

Zonemaps are created and refreshed for every SPU when you Generate statistics, Nzload operation, Insert, update operations, Nzreclaim operation.

Question 11. What Is Generate Statistics And Generate Express Statistics Or What Is The Difference Between Generate Statistics And Generate Express Statistics?

Answer :

Generate statistics is used to gather statistics about each table column’s proportion of duplicate values, minimum values, maximum values, null values, unique values and updates the system catalog tables.

The difference between ‘generate statistics’ and ‘generate express statistics’ is based on how the column uniqueness is calculates. The ‘generate express statistics’ calculates estimated dispersion values based on the sampling of rows in the table. ‘Generate express statistics’ uses approximation in generating the stats where as ‘generate statistics’ uses all the rows in the table.

Question 12. What Is The Use Of Creating Materialized Views?

Answer :

A materialized view reduces the width (number of columns) of data being scanned in the base table by creating a thin version (fewer columns) of the base table that contains a small subset of frequently queried columns.

Question 13. What Is The Distribution Of Materialized Views?

Answer :

A materialized view has the same distribution key as the base table.

Question 14. What Are The Limitations Of Materialized Views?

Answer :

• You cannot insert, update, delete or truncate a materialized view. Any changes on the base tables will reflect into materialized views.
• You can specify only one base table in the from clause.
• Base table can’t be a external table, system table or a temporary table.
• You cannot use a where clause in the materialized view.
• Expressions are not allowed as columns.

Question 15. What Are The Best Practices Of Creating Materialized Views?

Answer :

• Create materialized views with few columns which are frequently queried.
• Specify order by clause on the most restrictive columns (columns used in where clause).
• Periodically or manually refresh the materialized views.

Question 16. What Are The Partitioning Methods Available In Netezza?

Answer :

There are two partitioning methods available in netezza:

1. Random partitioning: Distributes the data randomly.
2. Hash Partitioning: Distributes the data on the specified columns.

Question 17. Up To How Many Columns You Can Specify In Distribute On Clause?

Answer :

You can specify up to four columns in the distribution clause.

Question 18. If You Did Not Specify Any Distribute On Clause While Creating A Table, What Distribution Netezza Uses?

Answer :

Netezza distributes the data on the first column and it uses Hash partitioning

Question 19. Can You Update The Columns Used In Distribution Clause?

Answer :

No, the column that is used in distribution clause cannot be used for updates.

Question 20. What Data Types Are Most Suited For The Columns Specified In Distribution Clause?

Answer :

Integer

Question 21. How Do You Redistribute A Table?

Answer :

Use Create Table As (CTAS) to redistribute the data in a table. While creating the new table specify the distribute on clause to distribute the data on the new columns.

Question 22. If You Did Not Specify Any Distribution Clause, How The Create Table As (ctas) Will Distribute The Rows?

Answer :

CTAS will get distribution from the original table.

Question 23. How Do You Check The Rows In A Table Are Equally Distributed In All Spu’s Or Not?

Answer :

1. To check the distribution of rows run the following query
2. SELECT datasliceid, COUNT(*)FROM <table name> GROUP BY datasliceid

Question 24. What Is Collocated Join?

Answer :

When you join tables which are distributed on the same key and used these key columns in the join condition, then each SPU in netezza works 100% independent of the other, as the required data is available in itself. This type of joins is called collocated joins.

Question 25. When Netezza Redistributes A Table And When It Broadcasts A Table?

Answer :

Whenever it is not possible to do a collocated join, netezza either redistributes the tables or broadcasts the table. When the table is a small one, then netezza broadcasts the table. Otherwise netezza redistributes the table.

Question 26. How Do You Remove Logically Deleted Records?

Answer :

Whenever you delete a row in a table, it is not physically deleted. It is logically deleted by flagging the deletexid field in the table. NZRECLAIM utility is used to remove the logically deleted records.

Question 27. What Is Nzload?

Answer :

Nzload utility is used load data from a file into a table. It is used to load bulk data quickly and simultaneously rejects erroneous content.

Question 28. What Are The Ways To Load A Data From A Table Into A File?

Answer :

Create an external table.

Use nzsql utility with -o option.

Question 29. What Are The Different Ways To Load Data From A File Into A Table?

Answer :

Use nzload to load the data from a file into a table

Create an external table and then load the original table using the external table.

Question 30. How Netezza Updates A Row In A Table?

Answer :

Netezza logically deletes the original row by flagging the deletexid column with the current transaction id and inserts a new row with the updated values.

Question 31. What Data Type Works Best For Zone Maps?

Answer :

Zone maps work best for integer data types.

Question 32. Can We Update All Columns In A Netezza Table.?

Answer :

No, the column that is used in distribution clause cannot be used for updates. Remember, up to four columns can be used for distribution of data on SPU. From practical sense, updating distribution columns result in redistribution of data; the single most performance hit when large table is involved. This restriction makes sense.

Question 33. When No Distribution Clause Is Used While Creating A Table, What Distribution Is Used By Netezza?

Answer :

First column (same as in Teradata).

Question 34. Why Do You Prefer Truncate Instead Of Drop Table Command?

Answer :

Just empties data from table, keeping table structure and permission intact.

Question 35. Create Table As (ctas), Does It Distribute Data Randomly Or Based On Table On Which It Received Data?

Answer :

Response: Newly created table from CTAS gets distribution from the original table.

Question 36. Why Not Null Specification Is Better In Netezza?

Answer :

Specifying not null results in better performance as NULL values are tracked at rowheader level. Having NULL values results in storing references to NULL values in header. If all columns are NOT NULL, then there is no record header.

Question 37. What Constraints On A Table Are Enforced?

Answer :

Not null and default. Netezza does not apply PK and FK.

Question 38. Do We Need To Drop All Tables And Objects In That Database, Before Dropping A Database?

Answer :

No, drop database will take care of it.

Question 39. What Permission Will You Give To Connect To A Database?

Answer :

List.  Grant list, select on table to public (if logged into sales database, this allows all users to query tables in sales database).

Question 40. How Can We Give A Global Permission To User Joe So That He Can Create Table In Any Database.?

Answer :

Login into system database and give that permission to user by saying “grant create table to joe;”

Question 41. What Happens To Records That Are Loaded During Nzload Process, But Were Not Committed?

Answer :

They are logically deleted and administrator can run nzreclaim, we may also truncate table.

Question 42. Can A Group Become A Member Of Another Group In Netezza User Administration. Can We Use Same Group Name For Databases?

Answer :

In Netezza, public group is created automatically and every one is a memeber of this group by default. We can create as many groups and any user can be member of any group(s). Group can not be a member of another group. Group names, user names and database names are unique. That is, we can not have a database called sales and a group also called sales.

Question 43. Does Netezza Support Concurrent Update Of The Same Record?

Answer :

In case of conflict in which the same record is set for modification, Netezza rolls back recent transaction that is attempted on the same record, in fact same table. This is generally acceptable in DW environments. Netezza does support serialization transactions and does not permit dirty reads.

Question 44. When Are We Likely To Receive Incorrect (aggregate) Results?

Answer :

Very rarely a driver may return aggregated results that are still getting processed back to client. In this case, client may assume that calculation is complete, instead of updating with latest or final results. Obviously, driver has to wait for Netezza to complete operation on host computer, before delivering results.

Question 45. How Is Load Achieved In Netezza And Why Is That Quick / Fast?

Answer :

Loads by pass a few steps that typically a query would go through (a query goes through plan generation, optimization and transaction management). Loads are done in terms of “sets” and this set is based on underlying table structure (thus loads for two different tables are different as their sets are based on table structures). Data is processed to check format and distribution of records calculated very quickly (in one step), fills into ‘set’ structure and writes to storage structure. Storage also performs space availability and other admin tasks, all these operations go pretty quick (think of them as UNIX named pipes that streams data and SPU stores these records).

Question 46. Does Everything Gets Cached In Netezza (or Any Other Data Appliance)?

Answer :

Typically only schema and other database objects are cached in appliances. Data is not cached, in general. In most cases, data is not saved any where (in any cache or on host computer) and is streamed directly from SPU to client software.

Question 47. How Do You Deal With Historical Data, With Respect To Zone Maps?

Answer :

Sort data first, based on historical data (for example, date) and load this in using nzload.

Question 48. What Is A Zone Map?

Answer :

Zone map in Netezza is similar (concept wise) to partitions in Oracle. Netezza maintains map for data so that it does relies on zone map to pull only the range it is interested in. For example, if we need to pull out data from Jan 2009 till June 2009 from a table that is distributed on date column, zone map helps us to achieve this. Zone map is maintained by Netezza automagically, no user intervention needed. Zone mapping is done at a block (extent) level. Netezza has zone maps for all columns (not just distributed column) and includes information such as minimum, maximum, total number of records.

Question 49. Explain Fpga And How Is It Useful For Query Performance?

Answer :

FPGA: Field Programmable Gate Array (FPGA) is located on each SPU. Netezza is different from other architectures. Netezza can do a “hardware upgrade” through software by using FPGA. Hardware is reconfigured during install.

While reading data from disk, FPGA on each SPU also helps in ‘filtering’ unnecessary data before getting loaded into memory on each SPU. This way, FPGA does not overwhelm with all the data from disk.

Question 50. What Are 4 Environment Variables That Are Required. What Are Different States On Netezza?

Answer :

Environment variables: NZ_HOST, NZ_DATABASE, NZ_USER and NZ_PASSWORD

1. Online: Normal or usual state.
2. Stopped: Netezza will shutdown after completing current queries, no new queries allowed.
3. Offline: Waits for completion of current queries, new or queries in queue receive error.
4. Paused: Same as above, but no error displayed. Typically caused during Netezza bootup or startup.
5. Down: Just plain down, could be due to Netezza server problem or user initiated.