Category Archives: SQL


“filter(NULL IS NOT NULL)” in explain plan usually means what CBO found a conflict between your WHERE-clause predicates and declarative constraints on the tables involved. example – And if constraints are validated, Oracle does not physically touch the Object at … Continue reading

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Lost control file, So what?

Recently got involved in couple conversations regarding common “RULE OF THUMB” about backing up control file, and double backup of control file backup etc.. I guess its good thing to do, its does not take much space anyway, but if … Continue reading

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ORA-01555 on standby

If you see “ORA-01555: snapshot too old” from Select what is been run on Physical standby, but same select runs on primary just fine, and all relevant init.ora parameters and undo tablespace are exactly identical between primary and standby,- its odd. … Continue reading

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Top-N query

Term of “TOP-N” intuitively makes thinking what they query should run faster than just ORDERED query, as only N elements needed from it. But, as well known, not always.. This blog describes method how using proper index would optimize TOP-N  … Continue reading

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Index Clustering factor. One way how to avoid negative performance impact on joins which involves bad clustered indexes.

High Index Clustering factor is popular problem in SQL world. The most known solution for it – rebuild table ordering by columns what are in the index. But in real world, with table size growing exponentially its not always practical … Continue reading

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