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docs: blurb about msq union all #15223

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Oct 31, 2023
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docs: blurb about msq union all #15223

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docs: blurb about msq union all' && git push
317brian Oct 20, 2023
9ab4130
change list to paragraph
317brian Oct 25, 2023
9e7fcd2
Merge branch 'master' into clarify-msq-union-all
317brian Oct 27, 2023
2b5962f
Update docs/querying/sql.md
317brian Oct 27, 2023
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7 changes: 4 additions & 3 deletions docs/querying/sql.md
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Expand Up @@ -218,9 +218,10 @@ UNION ALL
SELECT COUNT(*) FROM tbl WHERE my_column = 'value2'
```

:::info
With top-level queries, you can't apply GROUP BY, ORDER BY, or any other operator to the results of a UNION ALL.
:::
Keep the following in mind when writing top-level UNION ALL queries:

- You can't apply GROUP BY, ORDER BY, or any other operator to the results of a UNION ALL.
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Can coalesce into a single paragraph as mentioned in the other comment

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- You can't apply GROUP BY, ORDER BY, or any other operator to the results of a UNION ALL.

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@317brian 317brian Oct 23, 2023 •
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This limitation predates my changes and seems to apply to any query engine. So does it no longer apply to non-MSQ queries?

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This limitation is still present, but as per the previous comment, we can make it cohesive by coalescing both of the limitations into one big para.

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Ok, if we want it in 1 paragraph, I'll need to rewrite the paragraph first. Starting with If you use the MSQ task engine for the query, sets the reader up to think that the whole paragraph is only about the MSQ task engine limitations

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9ab4130 has the new paragraph

- If you use the MSQ task engine for the query, the SQL planner attempts to plan the top-level UNION ALL as a table-level UNION ALL. Because of this, the query behaves the same as a table-level UNION ALL, meaning it has the same characteristics and limitations. If the planner can't plan the query as a table-level UNION ALL, the query fails.
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Suggested change
- If you use the MSQ task engine for the query, the SQL planner attempts to plan the top-level UNION ALL as a table-level UNION ALL. Because of this, the query behaves the same as a table-level UNION ALL, meaning it has the same characteristics and limitations. If the planner can't plan the query as a table-level UNION ALL, the query fails.
- If you use the MSQ task engine for the query, the SQL planner attempts to plan the top-level UNION ALL as a table-level UNION ALL. Therefore it has the same limitations as the table level union all - You can only union TableDataSources with simple projections - select columns and rename columns. Clauses like GROUP BY, ORDER BY, etc are not supported on operands of union.
The query isn't supported if the planner can't plan it as a table-level union all


### Table-level

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