功能函数

• hll_empty()

  描述：创建一个空的hll。

  返回值类型：hll

  示例：

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  openGauss=# select hll_empty(); hll_empty ------------------------------------------------------------ \x484c4c00000000002b05000000000000000000000000000000000000 (1 row)

• hll_empty(int32 log2m)

  描述：创建空的hll并指定参数log2m，取值范围是10到16。若输入-1，则采用内置默认值。

  返回值类型：hll

  示例：

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  openGauss=# select hll_empty(10); hll_empty ------------------------------------------------------------ \x484c4c00000000002b04000000000000000000000000000000000000 (1 row) openGauss=# select hll_empty(-1); hll_empty ------------------------------------------------------------ \x484c4c00000000002b05000000000000000000000000000000000000 (1 row)

• hll_empty(int32 log2m, int32 log2explicit)

  描述：创建空的hll并依次指定参数log2m、log2explicit。log2explicit取值范围是0到12，0表示直接跳过Explicit模式。该参数可以用来设置Explicit模式的阈值大小，在数据段长度达到2^log2explicit后切换为Sparse模式或者Full模式。若输入-1，则log2explicit采用内置默认值。

  返回值类型: hll

  示例：

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  openGauss=# select hll_empty(10, 4); hll_empty ------------------------------------------------------------ \x484c4c00000000001304000000000000000000000000000000000000 (1 row) openGauss=# select hll_empty(10, -1); hll_empty ------------------------------------------------------------ \x484c4c00000000002b04000000000000000000000000000000000000 (1 row)

• hll_empty(int32 log2m, int32 log2explicit, int64 log2sparse)

  描述：创建空的hll并依次指定参数log2m、log2explicit、log2sparse。log2sparse取值范围是0到14，0表示直接跳过Sparse模式。该参数可以用来设置Sparse模式的阈值大小，在数据段长度达到2^log2sparse后切换为Full模式。若输入-1，则log2sparse采用内置默认值。

  返回值类型：hll

  示例：

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  openGauss=# select hll_empty(10, 4, 8); hll_empty ------------------------------------------------------------ \x484c4c00000000001204000000000000000000000000000000000000 (1 row) openGauss=# select hll_empty(10, 4, -1); hll_empty ------------------------------------------------------------ \x484c4c00000000001304000000000000000000000000000000000000 (1 row)

• hll_empty(int32 log2m, int32 log2explicit, int64 log2sparse, int32 duplicatecheck)

  描述：创建空的hll并依次指定参数log2m、log2explicit、log2sparse、duplicatecheck。duplicatecheck取0或者1，表示是否开启该模式，默认情况下该模式会关闭。若输入-1，则duplicatecheck采用内置默认值。

  返回值类型：hll

  示例：

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  openGauss=# select hll_empty(10, 4, 8, 0); hll_empty ------------------------------------------------------------ \x484c4c00000000001204000000000000000000000000000000000000 (1 row) openGauss=# select hll_empty(10, 4, 8, -1); hll_empty ------------------------------------------------------------ \x484c4c00000000001204000000000000000000000000000000000000 (1 row)

• hll_add(hll, hll_hashval)

  描述：把hll_hashval加入到hll中。

  返回值类型：hll

  示例：

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  openGauss=# select hll_add(hll_empty(), hll_hash_integer(1)); hll_add ---------------------------------------------------------------------------- \x484c4c08000002002b0900000000000000f03f3e2921ff133fbaed3e2921ff133fbaed00 (1 row)

• hll_add_rev(hll_hashval, hll)

  描述：把hll_hashval加入到hll中，和hll_add功能一样，只是参数位置进行了交换。

  返回值类型：hll

  示例：

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  openGauss=# select hll_add_rev(hll_hash_integer(1), hll_empty()); hll_add_rev ---------------------------------------------------------------------------- \x484c4c08000002002b0900000000000000f03f3e2921ff133fbaed3e2921ff133fbaed00 (1 row)

• hll_eq(hll, hll)

  描述：比较两个hll是否相等。

  返回值类型：bool

  示例：

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  openGauss=# select hll_eq(hll_add(hll_empty(), hll_hash_integer(1)), hll_add(hll_empty(), hll_hash_integer(2))); hll_eq -------- f (1 row)

• hll_ne(hll, hll)

  描述：比较两个hll是否不相等。

  返回值类型：bool

  示例：

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  openGauss=# select hll_ne(hll_add(hll_empty(), hll_hash_integer(1)), hll_add(hll_empty(), hll_hash_integer(2))); hll_ne -------- t (1 row)

• hll_cardinality(hll)

  描述：计算hll的distinct值。

  返回值类型：int

  示例：

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  openGauss=# select hll_cardinality(hll_empty() || hll_hash_integer(1)); hll_cardinality ----------------- 1 (1 row)

• hll_union(hll, hll)

  描述：把两个hll数据结构union成一个。

  返回值类型：hll

  示例：

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  openGauss=# select hll_union(hll_add(hll_empty(), hll_hash_integer(1)), hll_add(hll_empty(), hll_hash_integer(2))); hll_union -------------------------------------------------------------------------------------------- \x484c4c10002000002b090000000000000000400000000000000000b3ccc49320cca1ae3e2921ff133fbaed00 (1 row)