Neoview Query Guide (R2.3, R2.4, R2.5)

Data TypeToken DescriptionToken
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
LEFT_MERGE_JOIN Operator
The LEFT_MERGE_JOIN operator describes a portion of an execution plan that involves a merge
join. The LEFT_MERGE_JOIN differs from MERGE_JOIN only when it does not find a match in
the inner table. When no match is found, the left row is still returned, and the data from the right
table is set to null. See “MERGE_JOIN Operator” (page 27).
The LEFT_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
textLeft 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
Hybrid Hash Joins
In a hybrid hash join, the inner table is hashed on the join column into the memory of the process
doing the join. The outer table is read and the join column is hashed and matched against the
in-memory hash table. If the inner table is too large, it is subject to overflow to disk, which is
expensive and slow. The original row order is not guaranteed unless ordered hash joins are used.
HYBRID_HASH_JOIN Operator
The HYBRID_HASH_JOIN operator joins the data from two child tables. It creates a hash table
for the inner table and joins the outer table by hashing each outer row and looking for matches
in the hash table. This operator can overflow to disk when the inner table is too large to fit in
memory. Equijoins and cross-products are supported by this operator.
The HYBRID_HASH_JOIN 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
Operators and Description Fields 29