A primary index can be either partitioned or nonpartitioned. Primary indexes for join and hash indexes can also either be hash- or value-ordered, while primary indexes for all other table types are only hash-ordered.
The decision to define which of the two choices for a table depends on how its rows are most frequently accessed (see Single-Level Partitioning Case Studies). This topic describes the differences between the two types as well as their relative advantages and disadvantages.
Nonpartitioned Primary Indexes
When a table is created with a nonpartitioned primary index, its rows are hashed to the appropriate AMPs and stored there in row hash order.
Partitioned Primary Indexes
A partitioned primary index is defined to be either single-level or multilevel depending on how many partitioning expressions are defined for the table.
You can define partitioning for global temporary and volatile tables, for base tables (but not queue tables or hash indexes), and for noncompressed join indexes.
Optimal partitioning expressions are typically coded using CASE_N or RANGE_N expressions based on exact numeric, character or DateTime columns. DateTime expressions can include the BEGIN and END bound functions and the DATE, CURRENT_DATE, and CURRENT_TIMESTAMP functions.
The test value you specify with a RANGE_N function must result in a BYTEINT, SMALLINT, INTEGER, BIGINT, DATE, TIMESTAMP(n), TIMESTAMP(n) WITH TIME ZONE, CHARACTER, VARCHAR, GRAPHIC, or VARCHAR(n) CHARACTER SET GRAPHIC data type.
Support for the BEGIN and END bound functions also includes support for the IS [NOT] UNTIL_CHANGED and IS [NOT] UNTIL_CLOSED functions.
You can specify any valid Date or Time element for the IS [NOT] UNTIL_CHANGED and IS [NOT] UNTIL_CLOSED functions in partitioning expressions based on the CASE_N function.
You cannot specify multivalue compression for the partitioning columns of a partitioning expression.
Row-partitioning optimizes range queries while also providing efficient primary index join strategies. Analyze your range query optimization needs carefully because there are performance tradeoffs between specific range access improvements and possible decrements for primary index accesses and joins and aggregations on the primary index that occur as a function of the number of populated row partitions.
The partitioning maxima using different types of partitioning expressions are listed in the following table. These maxima apply to both row-partitioned tables and join indexes and to column-partitioned tables and join indexes (which may also have row partitioning).
| Partitioning Parameter | Maximum Value |
|---|---|
| Combined partitions for a table with 2-byte partitioning. | 65,535 |
| Combined partitions for a table with 8-byte partitioning. | 9,223,372,036,854,775,807 |
| Valid range for the number of partitioning levels for a table or join index with 2-byte partitioning. | 1 - 15 |
| Valid range for the number of partitioning levels for a table or join index with 8-byte partitioning. | 1 - 62 |
| RANGE_N Number of ranges plus NO RANGE [OR UNKNOWN] and UNKNOWN partitions, with 2-byte partitioning. |
65,535 |
| RANGE_N Number of ranges plus NO RANGE [OR UNKNOWN] and UNKNOWN partitions (whether or not specified), with 8-byte partitioning. |
9,223,372,036,854,775,807 |
| CASE_N Number of conditions. |
Bound by the request text size and other limits. |
When you have a partitioned table, the bits stored in the internal partition number of the rowID represent the combined partition number of the row, which is determined by combining the partitioning expressions of a PARTITION BY clause into a single expression. The expression that combines the partitioning expressions of multilevel partitioning into a single combined partition number is called a combined partitioning expression.
The terms internal partition number and combined partition number are defined as follows.
| Term | Definition |
|---|---|
| Internal partition number | A value Teradata Database calculates from the combined partition number. The internal partition number is used to number partitions internally and is stored in the rowID. The internal and combined partition numbers can be identical if no modification is needed. For single-level partitioning only, partitions for the NO RANGE [OR UNKNOWN], NO CASE [OR UNKNOWN], and UNKNOWN options are placed at fixed internal partitions, followed by partitions for range and conditions following. Modification is required to retain existing internal partition numbers after an ALTER TABLE request that drops or adds ranges or partitions. For a table without partitioning, the internal partition number is always zero. |
| Combined partition number | The value computed for the partitioning expressions for a row in a partitioned table. For a table without partitioning, the combined partition number is always 0. Combining the results for a table with single-level partitioning, the combined partition number is the same as the value of the single partitioning expression. When you specify the system-determined columns PARTITION or PARTITION#Ln in a request, Teradata Database converts the internal partition number stored in the rowID into the appropriate combined partition number or the partition number of the specified partitioning level and returns that value to the requestor. |
Teradata Database groups partitioned rows into partition groups on an AMP first by their internal partition number, then by row hash value within each internal partition (if there is a primary index, otherwise by the assigned hash bucket), and then by uniqueness value.
If a SELECT query specifies values for all the primary index columns, the AMP that contains those rows can be determined, and only one AMP needs to be accessed. If the query conditions are not specified on the partitioning columns, then each internal partition can be probed to find the rows based on the hash value, assuming there is no usable alternative index. If conditions are also specified on the partitioning columns, then row partition elimination might further reduce the number of partitions to be probed on that AMP (see Teradata Vantage™ - SQL Request and Transaction Processing, B035-1142 for information about row partition elimination).
If a SELECT query does not specify the values for all the primary index columns, an all-AMP, full-table scan is required for a table with no partitioning when there is no usable alternative index. However, with partitioning, if conditions are specified on the partitioning columns, row partition elimination might reduce what would otherwise be an all-AMP, full-table scan to an all-AMP scan of only the partitions that are not eliminated. The extent of row partition elimination depends on the partitioning expressions, the conditions specified in the query, and the ability of the Optimizer to recognize such opportunities.
Suppose a query only requests orders dated August, 2010. For a nonpartitioned table, the whole table is read to determine which rows are from August because the August rows are scattered throughout the table. For a table partitioned by month, on the other hand, row partition elimination can be used to exclude all the partitions that do not contain activity for August. Because all the August rows are grouped together in the partitioned table, it becomes possible to position directly to the first row for August, then read sequentially until a September row is found, at which point the system stops reading rows. This query requested about 1/12 of the rows so only about 1/12 of the partitioned table must be read. If you partition by day and change the query to make the date August 2nd, an even smaller subset of the table is all that must be read.
When rows for current day activity are inserted later tonight into the nonpartitioned table, they are scattered throughout the entire table, so the average hits per block value is very low. For the partitioned table on the other hand, the inserts are clustered in a smaller area. This provides a better locality of reference. If the partitioning is by week or by day, then the inserts are clustered even more tightly, and the hits per block value is very high.
If you want to delete the oldest month of data, the task is a full-table scan for the nonpartitioned table, but for the partitioned table the rows cluster together, so it is possible to delete them all with a high number of average hits per block. Transient journaling is not done for each of the rows, further enhancing performance when all the rows of a partition are deleted as the last action of a transaction.
You can see that if the table is partitioned on order date, there is no need for a NUSI on order date, and if you had one for the nonpartitioned table, you could drop it when you converted the primary index for the table to a partitioned primary index.
- A majority of the requests in the workload specify range predicates on some column, particularly a date column, of the candidate partitioned table.
- A majority of the queries in the workload specify an equality predicate on some column of the candidate partitioned table, and that column is either:
- Not the only column in the primary index column set
or
- Not a primary index column.
- Not the only column in the primary index column set
- The primary index is used for the following applications.
- Primarily or exclusively to distribute rows evenly across the AMPs.
- Rarely, if ever, to access rows or to join tables.
- To access rows using a condition specified on a column that is suitable for partitioning the table.
- The primary index, defined with the entire set of partitioning columns, if any, is used for the following applications.
- To distribute rows evenly across the AMPs.
- To access rows directly or as a table join condition.
- The primary index, defined without the entire set of partitioning columns, is used for the following applications.
- To distribute rows evenly across the AMPs.
- To access rows directly or as a table join condition.
In this case, you must pay particular attention to weighing and optimizing the performance tradeoffs the situation makes possible.
- Partitioned access by means of an equality constraint on all primary index and partitioning columns is as efficient as the same access made by means of a nonpartitioned primary index.
- Partitioned access on an equality constraint on the primary index columns and an equality or other constraint on the partitioning columns that limits access to a single partition, produces performance as efficient as that made with a nonpartitioned primary index.
- Partitioned access with a constraint on the partitioning columns can approach that of nonpartitioned primary index access when all the primary index columns are specified with equality constraints on an expression that does not reference any columns, depending on the extent of row partition elimination (an indirect measure of the number of internal partitions that must be probed before the desired row is found) required.
- Partitioned access via an equality condition on the primary index that neither includes all of the partitioning columns nor makes a constraint on the partitioning columns might not be as efficient as access by means of a nonpartitioned primary index depending on the number of internal partitions that contain data, but is generally more efficient than a full-table scan.
- Partitioned access when not all of the primary index columns are specified with equality constraints, but there are conditions specified on the partitioning columns that limit the number of internal partitions that must be scanned is more efficient than access by a nonpartitioned primary index, which requires a full-table scan.
- Inserting a large number of rows can be much faster than the same insert operation into an identical table that it is defined without partitioning.
To optimize load performance, the first partitioning expression of the table preferably should match the row arrival pattern of data, which is most often based on transaction date. In this case, for example, the first partitioning expression should be based on a DATE or TIMESTAMP column.
- Accessing rows using conditions on partitioning columns can be faster than the same access to the same rows when the table is defined with a nonpartitioned primary index.
The performance of SELECT, UPDATE, and DELETE operations is improved when queries specify conditions on the partitioning columns of the table because such predicates cause a higher average number of hits per data block.
The following design considerations are also important for this partitioned performance characteristic:- The degree of improvement in the performance of select, update, and delete operations is proportional to the extent of row partition elimination that can be realized (see
Teradata Vantage™ - SQL Request and Transaction Processing, B035-1142 for details about row partition elimination).
In other words, the more internal partitions that can be excluded by conditions specified on the partitioning columns for a partitioned table, the better.
- Performance improvements are generally greater as a function of how finely the granularity is defined for the internal partitions.
- The degree of improvement in the performance of select, update, and delete operations is proportional to the extent of row partition elimination that can be realized (see
Teradata Vantage™ - SQL Request and Transaction Processing, B035-1142 for details about row partition elimination).
- Depending on the number of internal partitions that contain data, accessing rows by means of an equality constraint on their primary index can be slower for a partitioned table than for the equivalent nonpartitioned table when you neither include all of the partitioning columns in the primary index definition nor specify a constraint on those columns in the query.The following design considerations are important for this partitioned performance characteristic:
- The more coarse the granularity of the partitioning you define, or the more internal partitions eliminated, the less performance degradation you are likely to experience.
- Defining an appropriate secondary index on the partitioned table can sometimes minimize the performance degradation of primary index access.
- Joins can be different for partitioned and nonpartitioned tables that are otherwise equivalent, but the effect of the different join strategies that arise cannot be predicted easily in many cases.
The join plan the Optimizer pursues depends on collected statistics, dynamic-AMP samples, and derived statistics. The usual recommendation applies here: check EXPLAIN reports to determine the best way to design your indexes to achieve the optimal join geography.
The following design considerations are important for this partitioned performance characteristic:- Primary index-to-primary index joins are more likely to generate different join plans than other partitioned-nonpartitioned join comparisons.
To minimize the potential for performance issues in making primary index-to-primary index joins, consider the following guidelines:
Partition the two tables identically if possible.
A coarser granularity of partitions is likely to be superior to a finer partition granularity.
- Examine your EXPLAIN reports to determine which join methods the Optimizer is selecting to join the tables.
Rowkey-based merge joins are generally better than joins based on another family of join methods.
- Efficient row partition elimination can often convert joins that would otherwise be poor performers into good performers.
- The most likely candidate for poor join performance is found when you are joining a partitioned table with a nonpartitioned table, the partitioned table partitioning column set is not defined in the nonpartitioned table, there are no predicates on the partitioned table partitioning column set, and there are many internal partitions defined for the partitioned table.
- Primary index-to-primary index joins are more likely to generate different join plans than other partitioned-nonpartitioned join comparisons.
- The need for secondary indexes is often different for partitioned and nonpartitioned tables that are otherwise equivalent.
Several opposing scenarios present themselves for resolving the issues that might arise from the existence or absence of secondary indexes:
| You probably should … | IF … |
|---|---|
| drop an existing secondary index on the partitioning column set of a partitioning expression | it is not required to enforce the uniqueness of the partitioning column set of a partitioning expression. In practice, it is unlikely that uniqueness would be required in this case. This has the following benefits:
|
| add a new secondary index on the primary index column set | one or more columns of the partitioning column set is not also a member of the primary index column set and the primary index values are unique. In this situation, you must define a USI on the primary index column set to enforce its uniqueness because you cannot define a partitioned primary index to be unique unless its definition contains all of the partitioning columns. In some cases, the addition of the USI results in worse performance. |
- Would the proposed workloads against the table be better supported by a partitioning expression based on a CASE_N or RANGE_N expression?
- Should the partitioning expression specify a NO CASE, NO CASE OR UNKNOWN, NO RANGE, NO RANGE OR UNKNOWN, or UNKNOWN option?
If the test value in a RANGE_N expression can never be null, there is no need for an UNKNOWN or NO RANGE OR UNKNOWN partition.
If a condition in a CASE_N expression can never be unknown, there is no need for an UNKNOWN or NO CASE OR UNKNOWN partition.
- Should the table be partitioned on only one level or on multiple levels?
- If the partitioning expression specifies CURRENT_DATE functions, CURRENT_TIMESTAMP functions, or both, how should the expression be configured to minimize problems deriving from reconciling to a new current date or current timestamp value?
- The query workloads that will be accessing the partitioned table.
This factor must be examined at both the specific, or particular, level and at the general, overall level.
- Performance
- Does a nonpartitioned table perform better than a partitioned table for the given workload and for particularly critical queries?
- Is one partitioning strategy more high-performing than others?
- Do other indexes such as USIs, NUSIs, join indexes, or hash indexes improve performance?
- Does a partitioning expression cause significant row partition elimination for queries to occur or not?
- Access methods and predicate conditions
- Is access to the table typically made by primary index, secondary index, or some other access method?
- Do typical queries specify an equality condition on the primary index and include the complete partitioning column set?
- Do typical queries specify a non-equality condition on the primary index or the partitioning columns?
- Join strategies
- Do typical queries specify an equality condition on the primary index column set (and partitioning column set if they are not identical)?
- Do typical queries specify an equality condition on the primary index column set but not on the partitioning column set?
- Do typical query conditions support row partition elimination?
- Data maintenance
- What are the relative effects of a partitioned table versus a nonpartitioned table with respect to the maintenance workload for the table?
- If you must define a USI on the primary index to make it unique, how much additional maintenance is required to update the USI subtable?
- Frequency and ease of altering partitions
- Is a process in place to ensure that ranges are added and dropped as necessary?
- Does the partitioning expression permit you to add and drop ranges?
- If the number of rows moved by dropping and adding ranges causes large numbers of rows to be moved, do you have a process in place to instead create a new table with the desired partitioning in place, then INSERT … SELECT or MERGE the source table rows into the newly created target table with error logging?
- If you want to delete rows when their partition is dropped, have you specified NO RANGE, NO RANGE OR UNKNOWN, or UNKNOWN partitions?
- Backup and restore strategies.
See Teradata® Archive/Recovery Utility Reference, B035-2412 for details of how partitioning might affect your archive and restore strategies.
See Single-Level Partitioning Case Studies for a set of design scenarios that evaluate the performance effects of several different partitioned primary indexes.
See Single-AMP Queries and Partitioned Tables and All-AMP Tactical Queries and Partitioned Tables for specific design issues related to partitioned table support for tactical queries.
Partitioning Expression Data Type Considerations
The partitioning expressions you can define for a partitioned table have certain restrictions regarding the data types you can specify within them and with respect to the data type of the result of the function.
The following table summarizes these restrictions.
| Data Type | PARTITION BY | ||
|---|---|---|---|
| RANGE_N | CASE_N | Expression | |
|
N | N | N |
| BIGINT | Y | X | I |
| BLOB | N | N | N |
| BYTE | X | X | X |
| BYTEINT | Y | X | I |
| CHARACTER | Y | X | I |
| CLOB | N | N | N |
| DATE | Y | X | I |
|
X | X | I |
|
X | X | I |
| GRAPHIC | N | X | N |
| INTEGER | Y | X | Y |
| INTERVAL YEAR | X | X | I |
| INTERVAL YEAR TO MONTH | X | X | X |
| INTERVAL MONTH | X | X | I |
| INTERVAL DAY | X | X | I |
| INTERVAL DAY TO HOUR | X | X | X |
| INTERVAL DAY TO SECOND | X | X | X |
| INTERVAL SECOND | X | X | X |
| LONG VARCHAR | Y | X | I |
| LONG VARCHAR CHARACTER SET GRAPHIC | N | N | N |
| PERIOD The BEGIN and END bound functions are valid in a partitioning expression when they are defined on a valid PERIOD data type column and the result can be cast implicitly to a numeric data type. |
N | X | N |
| SMALLINT | Y | X | I |
| TIME | X | X | X |
| TIME WITH TIME ZONE | X | X | X |
| TIMESTAMP | Y | X | X |
| TIMESTAMP WITH TIME ZONE | Y | X | X |
| UDT | N | N | N |
| VARBYTE | X | X | X |
| VARCHAR | Y | X | I |
| VARCHAR(n) CHARACTER SET GRAPHIC | N | N | N |
|
N | N | N |
The following table explains the abbreviations used in the previous table.
| Symbol | Definition |
|---|---|
| I | Valid for a partitioning expression. If the type is also the data type of the result, then it must be such that it can be cast to a valid INTEGER or BIGINT value. |
| N | Not valid for a partitioning expression. If the partitioning expression is defined using a CASE_N function, then this type is not valid for the CASE_N condition. |
| X | Valid for a partitioning expression, but cannot be the data type of the result of the expression. If the partitioning expression is defined using a CASE_N function, then this type is valid for the CASE_N condition. |
| Y | Valid for a partitioning expression and valid as the data type of the result of the partitioning expression. |
Column Partitioning a Table or Join Index
Besides partitioning tables or join indexes on their rows, you can also partition them on their columns. Column partitioning enables Teradata Columnar.
Column partitioning is a physical database design choice that is not suitable for all workloads. For example, column partitioning is usually not suitable for workloads that often select both a significant number of rows and project many columns from a table. However, column partitioning might be suitable if a request selects a significant number of rows, but projects only a few columns, or conversely, if a request projects many columns, but only selects a small number of rows.
Column partitioning can be suitable for the case where both a small number of rows are selected and only a few columns are projected. See Column Partitioning for more information about how table and join indexes can be partitioned on their columns and how column-partitioned tables and join indexes can be applied optimally.
A column-partitioned table or join index can also have 1 or more row partitioning levels.