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Editorial Team
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Asked: 2026-05-12T17:17:17+00:00

I have the following query which obtains transactions from the transactions table and transaction

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I have the following query which obtains transactions from the transactions table and transaction detail. Both tables have a big amount of entries, so this query takes a while to return results. 

SELECT * FROM transactions t LEFT JOIN transac_detail tidts ON (tidts.id_transac = t.id);

However, I’m more worried because of the fact that Oracle does a full table scan on both tables, according to explain plan, even though t.id and tidts.id_transac have indexes.

Is there any way to optimize this without touching table structure?

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1 Answer

  1. Editorial Team

    I don’t think it’s true that the SQL neccessarily would be best served with full table scans. This would really be most obviously true where there is a foreign key between the two, but even then there could be exceptions.

    I think the key question is this: “what proportion of the rows from each table are expected to be included in the result set?”. If the answer is “100% from each” then you have a clear case for full table scans (and a hash join).

    However consider the case where tables A and B are joined, with table A containing 5 rows with a foreign key to table B (the parent) containing a million rows. Obviously here you’d look for a full scan of A with a nested loop join to B (the join column on table B would be indexed because it would have to be a primary or unique key).

    In the OP’s case though it looks like you’d expect 100% rows to be returned from each table. I’d expect to see a full scan of both tables, and a hash join with TRANSACTIONS(probably the smaller table) being accessed first and built into the hash table. This would be the optimum join method, and I’d just be looking out for a situation where TRANSACTIONS is too big for a single-pass hash join. If the join spills to disk then that could be a performance worry and you’d have to look at increasing the memory allocation or equi-partitioning the two tables to reduce the memory requirement.

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