FaceBook Share. You can specify the correspondence between update links and reject links in the link order. The input data will be partitioned by key to ensure that records with the same key value are located in the same partition and processed by the same node, so only a few rows are fetched at a time, and the required memory is small.
The mismatched records will be placed in the data output specified by reject. The search operation is based on the search key column of the reference table. The lookup key column is defined in lookup. The search data and reference data will be read into the memory, so the memory needs to be large. There is no need to sort the data, but it should be noted that the partitioning method of the lookup table is the same as the reference table or all partitioning methods are used.
Write My Article. Download article as html. Doing outer joins left, right, full outer. Joining multiple tables with the same keys. Use the Merge stage when: Multiple update and reject links are needed e.
Lookup Stage Key Points The Lookup stage has a reference link, a single input link, a single output link and a single rejects link. It does not required data on the input link or reference link to be sorted. Lookup stage is a in-memory processing stage. Large look up table will result in the job failure if DataStage engine server runs out of memory.
The Key column names in main and lookup tables do not need to be the same as you map them in the stage. Make sure to select the right Lookup Stage Conditions see Example step 3. There are 4 options: Continue, Drop, Fail and Reject.
Continue: When the lookup table does not have the value appears in the main table, it will assign null values to the lookup table columns. In another word, this option works like Left Join. Drop: When the lookup table does not have the value appears in the main table, it will drop the row all together. In another word, this option works like Inner Join. Fail: When the lookup table does not have the value appears in the main table, the job will fail. Someone was saying that join does not support more than two input , while merge support two or more input one master and one or more update links.
I will say, that is highly incomplete information. The fact is join does support two or more input links left right and possibly intermediate links. But, yes, if you are tallking about full outer join then more than two links are not supported.
Coming back to main question of difference between Join and Merge Stage, the other significant differences that I have noticed are:. Merge has as many reject link as the update links if there are n-input links then 1 will be master link and n-1 will be the update link.
Join There are various ways in which data is being selected. Merge Data in Master record and update records are merged only when both have same value for the merge key columns. Please Turn OFF your ad blocker.
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