Neoview Query Guide (R2.2)

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Merge Joins

In a merge join, both tables are required to be sorted on the join column. A buffer of rows is read

from the inner and outer tables, and a row from the outer table is used to match inner table rows

in a match-merge pattern.

MERGE_JOIN Operator

The MERGE_JOIN operator describes a portion of an execution plan that involves a merge join.

This operator joins the data from its two child operators. The data streams from both children

must be in the same order. The operator joins all matching rows from each data stream. The

MERGE_JOIN operator works only with equijoins.

The MERGE_JOIN operator has two child operators. The description field for this operator

contains:

Data TypeToken DescriptionToken

integerA sequential number assigned to the fragment. 0 is always master

executor and 1 is reserved for the Explain plan. Numbers 2 to n will be

ESP or DAM fragments.

fragment_id

integerThe fragment_id for the parent fragment of the current fragment. Value

is (none) for master executor.

parent_frag

textmaster, ESP, or DAMfragment_type

textInner, union, or natural joinjoin_type

textName of join method: mergejoin_method

text1 for Type1 or 2 for Type2, depending on parallel join algorithmparallel_join_type

expr(text)Expression of the join predicatemerge_join_predicate

expr(text)Expression of the WHERE clause that is not included in the

merge_join_predicate or in a selection_predicates of any children

selection_predicates

MERGE_SEMI_JOIN Operator

The MERGE_SEMI_JOIN operator returns one row for the first match it finds in the inner table.

Conversely, MERGE_JOIN returns a row for all matches in the inner table. See “MERGE_JOIN

Operator” (page 40).

The MERGE_SEMI_JOIN operator has two child operators. The description field for this operator