Built-in Functions

HyperLogLog (HLL) has a series of built-in functions for internal data processing. Generally, users do not need to know how to use these functions. For details, see Table 1.

**Table 1** Built-in Functions
Function	Function
hll_in	Receives hll data in string format.
hll_out	Sends hll data in string format.
hll_recv	Receives hll data in bytea format.
hll_send	Sends hll data in bytea format.
hll_trans_in	Receives hll_trans_type data in string format.
hll_trans_out	Sends hll_trans_type data in string format.
hll_trans_recv	Receives hll_trans_type data in bytea format.
hll_trans_send	Sends hll_trans_type data in bytea format.
hll_typmod_in	Receives typmod data.
hll_typmod_out	Sends typmod data.
hll_hashval_in	Receives hll_hashval data.
hll_hashval_out	Sends hll_hashval data.
hll_add_trans0	Works similar to hll_add, and is used on the first phase of DNs in distributed aggregation operations.
hll_union_trans	Works similar to hll_union, and is used on the first phase of DNs in distributed aggregation operations.
hll_union_collect	Works similar to hll_union, and is used on the second phase of CNs in distributed aggregation operations to summarize the results of each DN.
hll_pack	Is used on the third phase of CNs in distributed aggregation operations to convert a user-defined type hll_trans_type to the hll type.
hll	Converts a hll type to another hll type. Input parameters can be specified.
hll_hashval	Converts the bigint type to the hll_hashval type.
hll_hashval_int4	Converts the int4 type to the hll_hashval type.