PFADD

The PFADD command in DiceDB is used to add elements to a HyperLogLog data structure. HyperLogLog is a probabilistic data structure used for estimating the cardinality of a set, i.e., the number of unique elements in a dataset.

Syntax

PFADD key element [element ...]

Parameters

Parameter	Description	Type	Required
key	The name of the HyperLogLog data structure. If it does not exist, a new one is created.	String	Yes
element	One or more elements to add to the HyperLogLog. Multiple elements can be specified, separated by spaces.	String	Yes

Return values

Condition	Return Value
At least one internal register was altered	1
No internal register was altered	0

Behaviour

• The command first checks if the specified key exists.
• If the key does not exist, a new HyperLogLog data structure is created.
• If the key exists but is not a HyperLogLog, an error is returned.
• The specified elements are added to the HyperLogLog, and the internal registers are updated based on the hash values of these elements.
• The HyperLogLog maintains an estimate of the cardinality of the set using these updated registers.

Errors

1. Wrong type error:

   • Error Message: (error) WRONGTYPE Key is not a valid HyperLogLog string value
   • Occurs when trying to use the command on a key that is not a HyperLogLog.

2. Syntax error:

   • Error Message: (error) wrong number of arguments for 'pfadd' command
   • Occurs when the command syntax is incorrect or missing required parameters.

Example Usage

Basic Usage

Adding a single element to a HyperLogLog:

127.0.0.1:7379> PFADD myhyperloglog "element1"
(integer) 1

Adding Multiple Elements

Adding multiple elements to a HyperLogLog:

127.0.0.1:7379> PFADD myhyperloglog "element1" "element2" "element3"
(integer) 1

Checking if the HyperLogLog was Modified

If the elements do not alter the internal registers:

127.0.0.1:7379> PFADD myhyperloglog "element1"
(integer) 0

Invalid Usage

Attempting to add elements to a key that is not a HyperLogLog:

127.0.0.1:7379> SET mykey "notahyperloglog"
OK
127.0.0.1:7379> PFADD mykey "element1"
(error) WRONGTYPE Key is not a valid HyperLogLog string value

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