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docs(macros): mention interpreting EXPLAIN output for SQLite

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Signed-off-by: Austin Bonander <austin@launchbadge.com>
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Austin Bonander 2020-07-23 20:19:08 -07:00
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1 changed files with 6 additions and 6 deletions
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src/macros.rs
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@ -101,7 +101,7 @@
/// In most cases, the database engine can tell us whether or not a column may be `NULL`, and
/// the `query!()` macro adjusts the field types of the returned struct accordingly.
///
/// For Postgres and SQLite, this only works for columns which come directly from actual tables,
/// For Postgres, this only works for columns which come directly from actual tables,
/// as the implementation will need to query the table metadata to find if a given column
/// has a `NOT NULL` constraint. Columns that do not have a `NOT NULL` constraint or are the result
/// of an expression are assumed to be nullable and so `Option<T>` is used instead of `T`.
@ -115,6 +115,10 @@
/// `NULL` which then depends on the semantics of what functions are used. Consult the MySQL
/// manual for the functions you are using to find the cases in which they return `NULL`.
///
/// For SQLite we perform a similar check to Postgres, looking for `NOT NULL` constraints
/// on columns that come from tables. However, for SQLite we also can step through the output
/// of `EXPLAIN` to identify columns that may or may not be `NULL`.
///
/// To override the nullability of an output column, [see below](#type-overrides-output-columns).
///
/// ## Type Overrides: Bind Parameters (Postgres only)
@ -227,15 +231,11 @@
/// .fetch_one(&mut conn)
/// .await?;
///
/// // For Postgres this would have been inferred to be `Option<i32>`, MySQL `i32`
/// // and SQLite it wouldn't have worked at all because we couldn't know the type.
/// // For Postgres this would have been inferred to be `Option<i32>`, MySQL/SQLite `i32`
/// assert_eq!(record.id, MyInt4(1));
/// # }
/// ```
///
/// As mentioned, this allows specifying the type of a pure expression column which is normally
/// forbidden for SQLite as there's no way we can ask SQLite what type the column is expected to be.
///
/// ## Offline Mode (requires the `offline` feature)
/// The macros can be configured to not require a live database connection for compilation,
/// but it requires a couple extra steps: