Question: I want to replace a
nested loops join with a hash join, and I need the syntax
for a hash join hint. How do I add a hash join hint to
my SQL?
Answer:
The use_hash hint is used to force a hash join in
Oracle SQL and you add the use_hash hint as follows:
select
/*+ use_hash */
col1 . . . ;
Please note that you must have enough PGA RAM to enable
hash joins. If yur database does not have enough RAM
to allow Oracle to perform a hash join, then the hash join
hint will be ignored because Oracle does not have the
computing resources.
The
use_hash hint requests a hash join against the
specified tables. Essentially, a hash join is a technique
whereby Oracle loads the rows from the driving table (the
smallest table, first after the where clause) into a
RAM area defined by the hash_area_size session
parameter, but the PGA areas regions are specific to your
release. In 12c the parameter is memory_target
if you are using automatic memory management.
Oracle
then uses a hashing technique to locate the rows in the
larger second table. As I mention in Chapter 10, a hash join
is often combined with parallel query in cases where both
tables are very large.
The following query is an example of
a query that has been hinted to force a hash join with
parallel query:
select /*+
use_hash(e,b) parallel(e, 4) parallel(b, 4) */
e.ename,
hiredate,
b.comm
from
emp e,
bonus b
where
e.ename = b.ename
;
Here is the execution plan for the hash join.
Note that both tables in this join are using parallel query
to obtain their rows:
OPERATION
----------------------------------------------------------------------
OPTIONS OBJECT_NAME POSITION
------------------------------ ----------------------------
----------
SELECT STATEMENT
3
HASH JOIN
1
PARALLEL_TO_SERIAL
TABLE ACCESS
FULL EMP 1
PARALLEL_TO_PARALLEL
TABLE ACCESS
FULL BONUS 2
Hash joins are often faster than
nested loops joins, especially in cases where the driving
table is filtered into a small number of rows in the query's
where clause.
Enabling Your Database to
Accept the use_hash Hint
The use_hash hint is very
finicky, and there are many conditions that must be
satisfied. It is not uncommon to find that a use_hash
hint is ignored and here are some common causes of this
problem.
-
Check initialization parameters
Make sure that you have the proper settings for
optimizer_index_cost_adj, hash_multiblock_io_count,
optimizer_max_permutations, and
hash_area_size. You can see Chapter 16 for details
on setting these parameters.
-
Verify driving table Make
sure that the smaller table is the driving table (the
first table in the from clause). This is because
a hash join builds the memory array using the driving
table.
-
Analyze CBO statistics Check
that tables and/or columns of the join tables are
appropriately analyzed.
-
Check for skewed columns
Histograms are recommended only for non-uniform column
distributions. If necessary, you can override the join
order chosen by the cost-based optimizer using the
ordered hint.
-
Check RAM region Ensure that
hash_area_size is large enough to hold the
smaller table in memory. Otherwise, Oracle must write to
the TEMP tablespace, slowing down the hash join.
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