A new capability of the optimizer is to use a hash algorithm to execute outer joins. Hence, oracle converts the outer join into an inner join. Hence, outer join is a waste of energy in that query. In this, oracle does a scan of the inner table, builds a ram hash table, and then probes for matching rows from the outer table. The right outer join is the opposite of the left outer join. Then it scans the larger table, probing the hash table to find the joined rows. I think thats the reason this query is consuming very high temp 87gb, can anyone help me explain this.
A statements execution plan is the sequence of operations oracle performs to run the statement. The end result is about 100 rows that contain aggregated data from selected rows from tables a and b. Wait total waited waited sqlnet message to client 1 0. Using this new hash outer join can result in a 50% reduction in logical io. Disabling hash joins oracle consulting, oracle support. The explain plan statement displays execution plans chosen by the oracle optimizer for select, update, insert, and delete statements. The row source tree is the core of the execution plan. Another type of join is called an oracle right outer join. The main usage of right join is hash outer, hash semi and hash anti join. Outer only differs from the inner in that there might be null filler values on one side or the other after the join. The cbo will only choose a hash join if you have allocated oracle enough ram area in which to perform the hash join.
Next, assume table2 contains a row with a null value in column2. Query optimizer uses the smaller of the two tablesdata sources to build a hash table on the join key in memory. Yes, the input tablet1 is located right, not left, thus named right join. Ive written notes about the different join mechanisms in the past but such things are always worth revisiting, so heres an accumulated bundle of comments about hash joins a hash join takes two inputs that in most of the oracle literature are referred to as the build table and the probe table these rowsources may be extracts from real tables or indexes, or might. It is categorized in left outer join, right outer join and full outer join by oracle 9i ansiiso 1999 standard. In a hash join, oracle uses ram memory to speed the join. This type of join returns all rows from the righthand table specified in the on condition and only those rows from the other table where the joined fields are equal join condition is met. The left outer join returns all the rows from the table on the left side of the join, along with the values from the right hand side, or nulls if a matching row doesnt exist. A full outer join is used to join two tables together and include nonmatching rows from each table. In a left outer join, the outer join operator is actually on the right of the equality operator. Select lumn, lumn from table1 full outer join table2 on lumn lumn. For more on the 11g hash join full outer execution plan, we.
If indeed for whatever reason a full outer join doesnt work for you, use all rows present in both t1 and t2 select from t1 inner join t2 union all all rows present in t1 but not in t2 select from t1 left outer join t2 where t2. If a child table is outer joined with a parent table and the latter is smaller than the former, oracle will just go ahead with hash outer join as discussed in the previous tip. Oracle database performs a join whenever multiple tables appear in the from clause of the query. The select list of the query can select any columns from any of these tables. All points raised in the previous section apply here also. Hash joins in a hash join, the oracle database does a fullscan of the driving table, builds a ram hash table, and then probes for matching rows in the other table. For certain types of sql, the hash join will execute faster than a nested loop join, but the hash join uses more ram resources. Left outer join returns all rows from the left first table specified in the on condition and only those rows from the right second table. In release 2, the fake column shows up in the unmatched rows. Oracle hash joins tips oracle consulting, oracle support.
An outer join is similar to equijoin but it gets also the nonmatched rows from the table. So when oracle decides that the selectivity of the part of the where conditions affecting one of the tables is good enough i. Dec 11, 2007 hence, oracle converts the outer join into an inner join. The oracle optimizer can perform a number of query transformations to improve the performance of sql. Oracle always uses hash join even when both tables are. Oracle guru laurent schneider notes that the new oracle 11g hash join full execution plan can result in less than 50% of the logical io consistent gets than a traditional join. Indices can still be used to match the records and to identifier missing records. It preserves the unmatched rows from the second right table, joining them with a null in the shape of the first left table. By removing the hint, execution plan changes from nested loops to. I found this after upgrading from oracle 11g release 1 to release 2.
In a right deep tree, the output of one hash join is the probe input to the next hash join. All of the hash joins are active at once and cannot share memory. Also return the rows from the outer joined where theres a match on the join key. Finally, the full outer join returns all rows from both tables, filling in.
Ive written notes about the different join mechanisms in the past but such things are always worth revisiting, so heres an accumulated bundle of comments about hash joins a hash join takes two inputs that in most of the oracle literature are referred to as the build table and the probe table these rowsources may be extracts from real tables or indexes, or might be. It then scans the other table in the join usually the larger one and probes the hash table for matches. It returns all valid rows from the table on the right side of the join keyword, along with the values from the table on the left side, or nulls if a matching row doesnt exist. Hash is whereby a hash code is used as an on lookup into the other rowset. Following is a simple test case to demonstrate the benefit of right join.
See the nested loop joins and hash joins sections of the performance tuning guide. A left outer join b is equivalent to b right outer join a, with the columns in a different order. Sep 07, 20 sudipta, if you read the whole comment again, paying most attention to the second half, you will see that ive said about the second plan that it looks as if oracle has produced it by examining the join order t2, t1 even though the hint told the optimizer to consider only the join order t1, t2 the point is that when considering the join order x, y for a hash join the optimizer is. Typically, when the full outer join condition between two tables is an equijoin, the hash full outer join execution method is possible, and oracle database uses it automatically. When we do begin probing, rows flow up the entire tree of hash joins without blocking. Mar 03, 2009 but as of oracle 10g, the join order can be swapped. The hash table and probe table was fixed in outer join and semi joins till 9i. A guide to assessing and resolving ora600 and ora7445 errors.
The fact that its an outer join, left or right, does not affect whether the query can benefit from indexes or not. Oracle is choosing to join table0 with the result of table1 x table2 using nested loops and takes hours. The hash join right semi, is a hash join plan used with where exists clauses to speed up the discovery of. An outer join means return all rows from one table. But as of oracle 10g, the join order can be swapped.
Dec 08, 2012 mostly these are represented by additional buffer sort operation that show up in the parallel version of an execution plan from 10g on pre 10g does the same internally but doesnt show in the execution plan, but there is a special case which is the hash join buffered operation that gets used with the hash data distribution of the join row. Now in 10g the hash table and probe table an be switched. Pictorial presentation of oracle right outer join example1. The ansi left outer join query v is equivalent to query u, as the where clause in v is applied after the left outer join is performed based on the condition specified in the on clause.
Mostly these are represented by additional buffer sort operation that show up in the parallel version of an execution plan from 10g on pre 10g does the same internally but doesnt show in the execution plan, but there is a special case which is the hash join buffered operation that gets used with the hash data distribution of the join row. Hash join full outer is included in the preceding plan step 3, indicating that the query uses the hash full outer join execution method. There are basically three types of join hash, merge and loop. Oracle performance manager charts and oracle sql analyze can automatically create and display explain plans for you. Ask tom join example, new vs old syntax oracle ask tom.
Nested loop joins are useful when small subsets of data are being joined and if the join condition is an efficient way of. This type of join returns all rows from the right hand table specified in the on condition and only those rows from the other table where the joined fields are equal join condition is met. For certain types of sql, the hash semi right join will execute faster than the hash semi join, and the former also uses less ram resources in that. This tutorial explains full outer join and uses in oracle. In a hash join, the database does a fullscan of the driving table, builds a ram hash table, and then probes for matching rows in the other table. Left outer join returns all rows from the left first table specified in the on condition and only those rows from the right. Merge is whereby rows are consumed from each side of the join in the correct sequence meaning that each side must be sequenced. If any two of these tables have a column name in common, then you must qualify. In hash semi right join, oracle uses ram memory to speed up the join. The best i can summarize it right now is that a left outer join against a query with a constant fake column and a where clause generates different results in the two oracle releases. Right join the secret of swapping join input dion cho.
In a hash join, oracle accesses one table usually the smaller of the joined results and builds a hash table on the join key in memory. In cases where a very small table is being joined to a large table, the oracle hash join will often dramatically speedup the query. Hash joins are the usual choice of the oracle optimizer when the memory is set up to accommodate them. In this case, hash outer right join will perform more efficiently than hash outer join and thats the reason oracle will opt for the former instead of the latter. Im trying to figure out whether i can hint it to use hash instead, but dont understand which hint and where to use. Nested loop joins are useful when small subsets of data are being joined and if the join condition is an efficient way of accessing the second table.
What is the difference between left and right outer join. Now in 10g oracle can switch the driving table for outer joins and semi joins based on the sizes of the row source. A join is a query that combines rows from two or more tables, views, or materialized views. Using the pubs schema, the following example shows the. Outer join when you must, if you do not need to outer join, dont do it of course ask them so how would you avoid this left outer join making it a right outer join doesnt count. A right outer join is one of the join operations that allow you to specify a join clause.
All of the hash joins must build their complete hash tables before we can begin probing. If the optimizer can see an opportunity to remove a table from a query, without affecting the resultset it will do so using join elimination. It must be kept in mind that an outer join returns the rows that match, plus the rows that do not match from one or both tables. It also helps you to understand the optimizer decisions, such as why the optimizer chose a nested loops join instead of a hash join, and lets you understand the performance of a query. A full outer join is such a join that performs a join between two tables that returns the results of an inner join as well as the results of a left and right outer join. We provide you with an oracle sample database named ot which is based on a global fictitious company that sells computer hardware including storage, motherboard, ram, video card, and cpu.
The oracle optimizer will convent an in clause to an exists clause internally, so they are identical in function, but the optimizer cannot perform a hash semi join if the subquery contains a distinct or union clause. Finally, the full outer join returns all rows from both tables, filling. A full outer join performs a join between two tables that returns the results of an inner join as well as the results of a left and right outer join. Again, tias, but the hash join means oracle is already using indices.
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