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docs: document DOLT_DIFF() secondary-index optimization #129

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9 changes: 9 additions & 0 deletions site/dolt/src/content/reference/sql/version-control/dolt-sql-functions.md
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Expand Up @@ -253,6 +253,15 @@ Note that the `DOLT_DIFF()` table function currently requires that argument valu

`DOLT_DIFF()` table function requires `SELECT` privilege on the specified table.

#### Performance

`DOLT_DIFF()` normally scans the table's primary index at both revisions, which means it always pays for a
full table scan even if you only care about a few rows. If a secondary index exists that covers every
non-generated column of the table — at both the `from_revision` and `to_revision` schemas — Dolt will use
that index instead, which can be significantly faster for large tables. An index that only covers part of
the table's columns (including one that only covers the primary key) is not eligible, since it can't be used
to detect every kind of change; add the missing columns to the index if you want `DOLT_DIFF()` to pick it up.

#### Options

```sql
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