Thursday, July 12, 2007

Can a change in execution plan change the results?

We've been using Oracle Domain indexes for a while now in order to search documents to get back a ranked order of things that meet certain criteria. The documents are releated to people, and we augment the basic text search with other filters and score metrics based on the 'people' side of things to get an overall 'suitability' score for the results in a search. Without giving too much away about the business I work with I can't really tell you much more about the product than that, but it's probably enough of a background for this little gem. We've known for a while that the domain index 'score' returned from a 'contains' clause is based not only on the document to which that score relates, but also on the rest of the set that is searched. An individual document score does not live in isolation, rather in lives in the context of the whole result set. No problem. As I say, we've known this for a while and so have our customers. Quite a while ago they stopped asking what the numbers mean and learned to trust them. However, today we realised something. Since the results are affected by the result set that is searched, this means that the results can be affected by the order in which the optimizer decides to execute a query. I can't give you a full end to end example, but I can assure you that the following is most definately the case on one of our production domain indexes (names changed, obviously): We have a two column table 'document_index', which contains 'id' and 'document_contents'. Both columns have an index. The ID being the primary key and the other being a domain index. The following SQL gives the related execution path: SELECT id, SCORE( 1 ) FROM document_index WHERE CONTAINS( document_contents, :1, 1 ) > 0 AND id = :2 SELECT STATEMENT TABLE ACCESS BY INDEX ROWID SCOTT.DOCUMENT_INDEX DOMAIN INDEX SCOTT.DOCUMENT_INDEX_IDX01 However, the alternative SQL gives this execution path: SELECT id, SCORE( 1 ) FROM document_index WHERE CONTAINS( document_contents, 'Some text', 1 ) > 0 AND id = :2 SELECT STATEMENT TABLE ACCESS BY INDEX ROWID SCOTT.DOCUMENT_INDEX INDEX UNIQUE SCAN SCOTT.DOCUMENT_INDEX_PK Normally, this kind of change in execution path wouldn't be a problem. But as stated earlier, the result of a score operation against a domain index is not just dependant on the individual records, but the context of the whole result set. The first execution provides you a score for the single document in the context of the all the documents in the table, the second gives you a score within the context of just that document. The scores are different. Now obviously, this is an extreme example, but more subtle examples will almost certainly exist if you combine the domain index lookups with any other where clause criteria. This is especially true if you're using literal values instead of bind variables in which case you may find the execution path changing between calls to the 'same' piece of SQL. My advice? Well, we're going to split our domain index look ups from all the rest of the filtering criteria, that way we can prepare the set of documents we want the search to be within and know that the scoring algorithm will be applied consistently.

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