The Impact of Skewing on Teradata demonstrated

Teradata professionals inevitably encounter the detrimental effects of data skew on query performance.

Yet, I believe only a handful of us know the precise extent of impact we should anticipate.

I sought out crucial data points and created a test scenario utilizing precise measurements to gain a comprehensive understanding.

I am an enthusiast of CPU utilization and disk activity, as opposed to run times, as these metrics consistently reflect accurate information. Obtaining this data is straightforward through DBC.DBQLOGTBL, unless restricted by database administrators in adverse scenarios.

I conducted tests on both SET and MULTISET tables to broaden the test scenario. The rationale behind incorporating this attribute at the table level was to:

Considering the impact of skewing on performance, wouldn’t it be necessary to impose duplicate row checks on a SET table, thereby increasing the workload on the AMPs affected by skewing?

The configuration of my test environment is as follows:

CREATE SET TABLE TABLE_NOT_SKEWED (A INTEGER, B INTEGER) PRIMARY INDEX (A) ;

CREATE MULTISET TABLE TABLE_NOT_SKEWED_MULTISET (A INTEGER, B INTEGER) PRIMARY INDEX (A) ;

CREATE SET TABLE TABLE_SKEWED (A INTEGER, B INTEGER) PRIMARY INDEX (B) ;

CREATE MULTISET TABLE TABLE_SKEWED_MULTISET (A INTEGER, B INTEGER) PRIMARY INDEX (B) ;

I generated both a skewed and unskewed table as both SET and MULTISET. I populated them with data, wherein Column A is nearly unique, and Column B contains only a handful of distinct values.

INSERT INTO TABLE_NOT_SKEWED  SELECT RANDOM(1,1000000), RANDOM(1,5) FROM  SYS_CALENDAR.CALENDAR;

INSERT INTO TABLE_NOT_SKEWED_MULTISET  SELECT RANDOM(1,1000000), RANDOM(1,5) FROM  SYS_CALENDAR.CALENDAR;

INSERT INTO TABLE_SKEWED  SELECT RANDOM(1,1000000), RANDOM(1,5) FROM  SYS_CALENDAR.CALENDAR;

INSERT INTO TABLE_SKEWED_MULTISET  SELECT RANDOM(1,1000000), RANDOM(1,5) FROM  SYS_CALENDAR.CALENDAR;

After careful analysis, I examined DBC.DBQLOGTBL:

SELECT   QUERYTEXT,  MAXAMPCPUTIME,  MINAMPCPUTIME,  MAXAMPIO,  MINAMPIO,  TOTALIOCOUNT, AND AMPCPUTIME  FROM DBC.DBQLOGTBL WHERE SESSIONID = ; — my SQL Assistant session

Below you can see the result of this test. Here are my observations:

SET TABLES

compared to the non-skewed table. This means the database system takes significantly longer to retrieve data from the skewed table than from the non-skewed table.

The most active AMP must perform 4000 times as many disk accesses as the least busy AMP while utilizing similar CPU seconds.

MULTISET TABLES

Multiset tables require fewer disk accesses than their set table counterparts because the expensive duplicate row check is omitted.

The skewed multiset table requires fewer disk accesses than the unskewed table by less than 50%. However, this does not necessarily indicate that it is superior.

Most rows are likely adjacent on a small number of AMPs, which reduces disk access by utilizing memory caching and optimizing block reading methods.

Unfortunately, only a limited number of AMPs are assigned to manage numerous rows, resulting in a bottleneck. Therefore, it would be unwise to be deceived by these figures as inserting into the table remains significantly slower than when undertaken by a single or a select few AMPs.

Use multiset tables when available to minimize table skew and reduce the negative effects on duplicate row checks.