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POHow to change update execution plan in Oracle?
 

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  1. POHow to change update execution plan in Oracle?
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    <p>I have a large table foo_large and a relatively small (a few hundred thousand rows) table foo_small. The large table has a primary-key column "id"; the small one also has "id" column that is a foreign key to foo_large. I want to update foo_small so that for every row its col_y has a value equal to col_x in the corresponding row of foo_large. The most straightforward way seems to be like this:</p> <pre><code>update foo_small sm set col_y = ( select col_x from foo_large where id = sm.id); </code></pre> <p>This is, however, very inefficient. For each row of foo_small, the corresponding row of foo_large is accessed by the index on its primary key. Though foo_small is small compared to foo_large, it still leads to several hundred thousand of index scans on that table. A better solution would be to hash foo_small in memory and perform a (possibly parallelized) full scan on foo_large once, updating the matching rows of foo_small encountered. I can do that in the following way:</p> <pre><code>update ( select /*+ ordered use_hash(lg) parallel(lg 2) */ sm.*, lg.col_x from foo_small sm, foo_large lg where sm.id = lg.id ) set col_y = col_x; </code></pre> <p>This query finishes within a minute. Unfortunately, it has another disadvantage: It requires that the user launching this query has a privilege to update not only foo_small, but also foo_large even though the latter table is not actually updated. Is there a solution to force the latter execution plan without updating a join? I know I could write a piece of procedural PL/SQL code with bulk fetch/update and probably keep most of the performance gain, but I suppose there must be a way to do it in a single query.</p> <p>Thanks in advance.</p>
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    1. USTomasz Żuk
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      1. POHow to change update execution plan in Oracle?
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      1. USJon Heller
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    1. COCan you hint `merge into foo_small sm using foo_large lg on (lg.id = sm.id) when matched then update set col_y = lg.col_x` to get a plan you want? It seems like that should not require anything more than select rights on foo_large.
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      1. USShannon Severance
    2. COLooks pretty clever; I'll try it tomorrow.
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      1. USTomasz Żuk
    3. CO@shannon OK, I've given the hints `/*+ leading(sm) full(lg) use_hash(lg) parallel(lg 2) */` and it goes with the efficient execution plan. Thanks!
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      1. USTomasz Żuk
 

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