Column Merge#

group column_merge

Functions

std::unique_ptr<cudf::table> merge(std::vector<table_view> const &tables_to_merge, std::vector<cudf::size_type> const &key_cols, std::vector<cudf::order> const &column_order, std::vector<cudf::null_order> const &null_precedence = {}, rmm::cuda_stream_view stream = cudf::get_default_stream(), rmm::device_async_resource_ref mr = cudf::get_current_device_resource_ref())#

Merge a set of sorted tables.

Merges sorted tables into one sorted table containing data from all tables. The key columns of each table must be sorted according to the parameters (cudf::column_order and cudf::null_order) specified for that column.

Example 1:
input:
table 1 => col 1 {0, 1, 2, 3}
           col 2 {4, 5, 6, 7}
table 2 => col 1 {1, 2}
           col 2 {8, 9}
table 3 => col 1 {2, 4}
           col 2 {8, 9}
output:
table => col 1 {0, 1, 1, 2, 2, 2, 3, 4}
         col 2 {4, 5, 8, 6, 8, 9, 7, 9}

Example 2:
input:
table 1 => col 0 {1, 0}
           col 1 {'c', 'b'}
           col 2 {RED, GREEN}


table 2 => col 0 {1}
           col 1 {'a'}
           col 2 {NULL}

 with key_cols[] = {0,1}
 and  asc_desc[] = {ASC, ASC};

 Lex-sorting is on columns {0,1}; hence, lex-sorting of ((L0 x L1) V (R0 x R1)) is:
 (0,'b', GREEN), (1,'a', NULL), (1,'c', RED)

 (third column, the "color", just "goes along for the ride";
  meaning it is permuted according to the data movements dictated
  by lexicographic ordering of columns 0 and 1)

  with result columns:

  Res0 = {0,1,1}
  Res1 = {'b', 'a', 'c'}
  Res2 = {GREEN, NULL, RED}

Throws:

  • cudf::logic_error – if tables in tables_to_merge have different number of columns

  • cudf::logic_error – if tables in tables_to_merge have columns with mismatched types

  • cudf::logic_error – if key_cols is empty

  • cudf::logic_error – if key_cols size is larger than the number of columns in tables_to_merge tables

  • cudf::logic_error – if key_cols size and column_order size mismatches

Parameters:

  • tables_to_merge – [in] Non-empty list of tables to be merged

  • key_cols – [in] Indices of left_cols and right_cols to be used for comparison criteria

  • column_order – [in] Sort order types of columns indexed by key_cols

  • null_precedence – [in] Array indicating the order of nulls with respect to non-nulls for the indexing columns (key_cols)

  • stream – CUDA stream used for device memory operations and kernel launches

  • mr – Device memory resource used to allocate the returned table’s device memory

Returns:

A table containing sorted data from all input tables

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