wehub-resource-sync
47 lines
1.9 KiB
Markdown
47 lines
1.9 KiB
Markdown
# Reactivity and Constraints
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## Reactive Updates
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Joined tables are fully reactive. When either source table receives an
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`update()`, the join is automatically recomputed and any `View` created from the
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joined table will reflect the new data. This includes:
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- Updates that modify existing rows in either source table.
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- New rows added to either source table that create new matches.
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- Chained joins — if a joined table is itself used as input to another join,
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updates propagate through the entire chain.
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## Duplicate Keys
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Like SQL, `join()` produces a cross-product for each matching key value. When
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multiple rows in the left table share the same key, each is paired with every
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matching row in the right table (and vice versa). The number of output rows for
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a given key is `left_count × right_count`.
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This behavior depends on whether the source tables are _indexed_:
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- **Unindexed tables** (no `index` option) — rows are appended, so duplicate
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keys accumulate naturally. Each `update()` appends new rows, which may
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introduce additional duplicates.
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- **Indexed tables** (`index` set to the join key) — each key appears at most
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once per table, so the join produces at most one row per key. Updates replace
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existing rows in-place rather than appending.
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## Read-Only
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Joined tables are read-only. Calling `update()`, `remove()`, `clear()`, or
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`replace()` on a joined table will throw an error. Data can only change
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indirectly, by updating the source tables.
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## Column Name Conflicts
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The left and right tables must not have overlapping column names (other than the
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join key). If a non-key column name appears in both tables, `join()` throws an
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error. Rename columns in your source data or use `View` expressions to avoid
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conflicts.
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## Source Table Deletion
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A source table cannot be deleted while a joined table depends on it. You must
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delete the joined table first, then delete the source tables.
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