ER/Studio - Embarcadero Technologies Product Documentation

USING ER/STUDIO > DEVELOPING THE PHYSICAL MODEL
• Partitioning Expression: (PARTITION BY expression) Available if you selected Primary. Enter an expression to
partition the data by a particular attribute, such as date by week, for some time. Partitioning the data enables you
to more efficiently retrieve summarized data for a particular week because you only have to read the data for that
week. Good candidates for a Partitioned Primary Index are tables where there is a high volume of daily inserts,
so there is a bias toward partitioning on the data date. Partitioning an index by product_code or agent_id can also
improve some queries. If you need more space than available in the text box to enter your expression, click More
to launch the Partition Expression Editor where you can input a multi-line expression. The following is an example
of a partitioning expression:
PARTITION BY RANGE_N(CAPTURE_DATE BETWEEN DATE '2007-11-30' AND DATE '2008-2-30'
EACH INTERVAL '1' MONTH)
Oracle 7 Index Options
• Unique: (UNIQUE) If selected, specifies that the index key contains no duplicate values and therefore every row
in the table is in some way unique. If not selected, a non-unique index is created that lets table rows have
duplicate values in the columns that define the index.
• No Sort: (NOSORT) If selected, specifies to not sort the data when the index is rebuilt.
Postgre SQL Index Options
• Unique: (UNIQUE) If selected, specifies that the index key contains no duplicate values and therefore every row
in the table is in some way unique. If not selected, a non-unique index is created that lets table rows have
duplicate values in the columns that define the index.
• Clustered: (CLUSTER) Select to create a clustered index. A clustered index specifies that the rows of the table
should be inserted sequentially on data pages, which generally requires less disk I/O to retrieve the data.
Generally, the clustered index should be on a column that monotonically increases, such as an identity column,
or some other column where the value is increasing, and is unique. This is especially true if the table is subject to
many INSERTS, UPDATES, and DELETES. Clustered indexes can greatly increase access speed, but usually only
where the data is accessed sequentially in the same or reverse order of the clustered index, or when a range of
items are selected.
Sybase Index Options
The options available depend upon the database platform version.
• Unique: (UNIQUE) If selected, specifies that the index key contains no duplicate values and therefore every row
in the table is in some way unique. If not selected, a non-unique index is created where table rows can have
duplicate values in the columns that define the index.
• Type: Sybase IQ uses HG or LF indexes to generate better or faster query plans to execute the query. The HG or
LF index may also be appropriate for a DATE, TIME, DATETIME, or TIMESTAMP column, if you are evaluating
equality predicates against the column. An LF index is also recommended, if you frequently use the column in the
GROUP BY clause and there are less than 1000 distinct values, for example, less than three years of dates.
• HG: Use an HG (High Group) index when the column will be used in a join predicate or the column has more
than 1,000 unique values. The High_Group index is commonly used for join columns with integer data types
because it handles GROUP BY efficiently. Each foreign key columns requires its own HG index; however, if a
join index exists, the same column cannot have both an explicitly created HG index and a foreign key
constraint.
• LF: LF is the fastest index in Sybase IQ. Use an LF (Low Fast) index when a column has less than 1,000
unique values or a column has less than 1,000 unique values and is used in a join predicate. This index is
ideal for columns that have a few unique values (under 1,000) such as sex, Yes/No, True/False, number of
dependents, and wage class.Never use an LF index for a column with 10,000 or more unique values. If the
table has less than 25,000 rows, use an HG index, which requires fewer disk I/O operations for the same
operation.
EMBARCADERO TECHNOLOGIES > ER/STUDIO® 8.0.3 USER GUIDE 173