Document aggregation pushdown in Oracle connector #11805
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Ping @wendigo .. the code change is in master .. ideally we get the docs in before the release as well.. |
| Pushdown is only supported for ``DOUBLE`` type columns with the | ||
| following functions: |
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Some such type limitations also apply to the functions above as well but they can vary based on type mappings and what the actual data-type is in remote system.
Longer term I would prefer to move docs to a direction where we clearly set the expectation that operations will be pushed down where possible without trying to overspecify those conditions since it's complex, will never get 100% accurate due to the amount of possible combinations and will change without much visibility.
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That sounds good .. the problem is that users test this stuff and then ask specific questions why something is and is not pushed down .. ideally the docs have the answer..
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The answer is if it doesn't work then it's unsupported. We obviously need a better way to communicate that for sure.
For a taste of complexity to know whether sum(x) will be pushed down:
- data type of x as Trino sees it
- the underlying data type in remote database
- any implicit casts that get added by engine (may vary based on query shape)
- the data type of the grouping columns used
- whether x is an expression or a plain column reference
- whether there are other operations that cannot be pushed down (this is unbounded list of things)
As an example of practical real world case where people don't really care about "when" things get pushed down is that sum() since the beginning has only been pushdown-able if the column is a bigint or a decimal but no one ever complained or asked about this. People generally care about performance - not exactly how that performance is achieved.
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Merging since it's accurate and the comment needs further discussion and applies to docs as whole instead of just this PR. |
Description
Add list of functions that can be pushed down to Oracle with aggregation pushdown.
Documentation fix.
Oracle connector.
Add details to Oracle connector documentation.
Related issues, pull requests, and links
Documentation
( ) No documentation is needed.
(x) Sufficient documentation is included in this PR.
( ) Documentation PR is available with #prnumber.
( ) Documentation issue #issuenumber is filed, and can be handled later.
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