Specifying DB parameters - Amazon Relational Database Service
DB parameter formulasDB parameter functionsBoolean DB parameter expressionsDB parameter log expressionsDB parameter value examples
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Specifying DB parameters

DB parameter types include the following:

  • Integer

  • Boolean

  • String

  • Long

  • Double

  • Timestamp

  • Object of other defined data types

  • Array of values of type integer, Boolean, string, long, double, timestamp, or object

You can also specify integer and Boolean parameters using expressions, formulas, and functions.

For the Oracle engine, you can use the DBInstanceClassHugePagesDefault formula variable to specify a Boolean DB parameter. See DB parameter formula variables.

For the PostgreSQL engine, you can use an expression to specify a Boolean DB parameter. See Boolean DB parameter expressions.

DB parameter formulas

A DB parameter formula is an expression that resolves to an integer value or a Boolean value. You enclose the expression in braces: {}. You can use a formula for either a DB parameter value or as an argument to a DB parameter function.

Syntax
{FormulaVariable}
{FormulaVariable*Integer}
{FormulaVariable*Integer/Integer}
{FormulaVariable/Integer}

DB parameter formula variables

Each formula variable returns an integer or a Boolean value. The names of the variables are case-sensitive.

AllocatedStorage

Returns an integer representing the size, in bytes, of the data volume.

DBInstanceClassHugePagesDefault

Returns a Boolean value. Currently, it's only supported for Oracle engines.

For more information, see Turning on HugePages for an RDS for Oracle instance.

DBInstanceClassMemory

Returns an integer for the number of bytes of memory available to the database process. This number is internally calculated by starting with the total amount of memory for the DB instance class. From this, the calculation subtracts memory reserved for the operating system and the RDS processes that manage the instance. Therefore, the number is always somewhat lower than the memory figures shown in the instance class tables in DB instance classes. The exact value depends on a combination of factors. These include instance class, DB engine, and whether it applies to an RDS instance or an instance that's part of an Aurora cluster.

DBInstanceVCPU

Returns an integer representing the number of virtual central processing units (vCPUs) used by Amazon RDS to manage the instance. Currently, it's only supported for the RDS for PostgreSQL engine.

EndPointPort

Returns an integer representing the port used when connecting to the DB instance.

TrueIfReplica

Returns 1 if the DB instance is a read replica and 0 if it is not. This is the default value for the read_only parameter in MySQL.

DB parameter formula operators

DB parameter formulas support two operators: division and multiplication.

Division operator: /

Divides the dividend by the divisor, returning an integer quotient. Decimals in the quotient are truncated, not rounded.

Syntax

dividend / divisor

The dividend and divisor arguments must be integer expressions.

Multiplication operator: *

Multiplies the expressions, returning the product of the expressions. Decimals in the expressions are truncated, not rounded.

Syntax

expression * expression

Both expressions must be integers.

DB parameter functions

You specify the arguments of DB parameter functions as either integers or formulas. Each function must have at least one argument. Specify multiple arguments as a comma-separated list. The list can't have any empty members, such as argument1,,argument3. Function names are case-insensitive.

IF

Returns an argument.

Currently, it's only supported for Oracle engines, and the only supported first argument is {DBInstanceClassHugePagesDefault}. For more information, see Turning on HugePages for an RDS for Oracle instance.

Syntax

IF(argument1, argument2, argument3)

Returns the second argument if the first argument evaluates to true. Returns the third argument otherwise.

GREATEST

Returns the largest value from a list of integers or parameter formulas.

Syntax

GREATEST(argument1, argument2,...argumentn)

Returns an integer.

LEAST

Returns the smallest value from a list of integers or parameter formulas.

Syntax

LEAST(argument1, argument2,...argumentn)

Returns an integer.

SUM

Adds the values of the specified integers or parameter formulas.

Syntax

SUM(argument1, argument2,...argumentn)

Returns an integer.

Boolean DB parameter expressions

A Boolean DB parameter expression resolves to a Boolean value of 1 or 0. The expression is enclosed in quotation marks.

Note

Boolean DB parameter expressions are only supported for the PostgreSQL engine.

Syntax
"expression operator expression"

Both expressions must resolve to integers. An expression can be the following:

  • integer constant

  • DB parameter formula

  • DB parameter function

  • DB parameter variable

Boolean DB parameter expressions support the following inequality operators:

The greater than operator: >

Syntax

"expression > expression"
The less than operator: <

Syntax

"expression < expression"
The greater than or equal to operators: >=, =>

Syntax

"expression >= expression"
"expression => expression"
The less than or equal to operators: <=, =<

Syntax

"expression <= expression"
"expression =< expression"
Example using a Boolean DB parameter expression

The following Boolean DB parameter expression example compares the result of a parameter formula with an integer. It does so to modify the Boolean DB parameter wal_compression for a PostgreSQL DB instance. The parameter expression compares the number of vCPUs with the value 2. If the number of vCPUs is greater than 2, then the wal_compression DB parameter is set to true.

aws rds modify-db-parameter-group --db-parameter-group-name group-name \
--parameters "ParameterName=wal_compression,ParameterValue=\"{DBInstanceVCPU} > 2\" "

DB parameter log expressions

You can set an integer DB parameter value to a log expression. You enclose the expression in braces: {}. For example:

{log(DBInstanceClassMemory/8187281418)*1000}

The log function represents log base 2. This example also uses the DBInstanceClassMemory formula variable. See DB parameter formula variables.

Note

Currently, you can't specify the MySQL innodb_log_file_size parameter with any value other than an integer.

DB parameter value examples

These examples show using formulas, functions, and expressions for the values of DB parameters.

Warning

Improperly setting parameters in a DB parameter group can have unintended adverse effects. These might include degraded performance and system instability. Use caution when modifying database parameters and back up your data before modifying your DB parameter group. Try out parameter group changes on a test DB instance, created using point-in-time-restores, before applying those parameter group changes to your production DB instances.

Example using the DB parameter function GREATEST

You can specify the GREATEST function in an Oracle processes parameter. Use it to set the number of user processes to the larger of either 80 or DBInstanceClassMemory divided by 9,868,951.

GREATEST({DBInstanceClassMemory/9868951},80)
Example using the DB parameter function LEAST

You can specify the LEAST function in a MySQL max_binlog_cache_size parameter value. Use it to set the maximum cache size a transaction can use in a MySQL instance to the lesser of 1 MB or DBInstanceClass/256.

LEAST({DBInstanceClassMemory/256},10485760)