Precision Functions
HLL supports explicit, sparse, and full modes. explicit and sparse excel when the data scale is small, and barely produce errors in calculation results. When the number of distinct values increases, full becomes more suitable, but produces some errors. The following functions are used to view precision parameters in HLLs.
hll_schema_version(hll)
Description: Checks the schema version in the current HLL.
Example:
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SELECT hll_schema_version(hll_empty()); hll_schema_version -------------------- 1 (1 row) |
hll_type(hll)
Description: Checks the type of the current HLL.
Example:
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SELECT hll_type(hll_empty()); hll_type ---------- 1 (1 row) |
hll_log2m(hll)
Description: Check the value of log2m of the current HLL. This value affects the error rate in calculating the number of distinct values by the HLL. The formula for calculating the error rate is as follows:
Example:
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SELECT hll_log2m(hll_empty()); hll_log2m ----------- 11 (1 row) |
hll_regwidth(hll)
Description: Checks the number of bits of buckets in a hll data structure.
Example:
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SELECT hll_regwidth(hll_empty()); hll_regwidth -------------- 5 (1 row) |
hll_expthresh(hll)
Description: Obtains the size of expthresh in the current HLL. An HLL usually switches from the explicit mode to the sparse mode and then to the full mode. This process is called the promotion hierarchy policy. You can change the value of expthresh to change the policy. For example, if expthresh is 0, an HILL will skip the explicit mode and directly enter the sparse mode. If the value of expthresh is explicitly set to a value ranging from 1 to 7, this function returns 2expthresh.
Example:
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SELECT hll_expthresh(hll_empty()); hll_expthresh --------------- (-1,160) (1 row) SELECT hll_expthresh(hll_empty(11,5,3)); hll_expthresh --------------- (8,8) (1 row) |
hll_sparseon(hll)
Description: Specifies whether to enable the sparse mode. 0 indicates off and 1 indicates on.
Example:
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SELECT hll_sparseon(hll_empty()); hll_sparseon -------------- 1 (1 row) |
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