Struct KeyProber 

pub struct KeyProber<'a, 't> { /* private fields */ }

Probes a string primary key column. Only supports utf8mb4_bin against CHAR/VARCHAR columns up to 768 characters. Enforcement is deferred to the caller. There may be other collations we can support, but we should do more validation.

Implementations

impl<'a, 't> KeyProber<'a, 't>

pub fn new( tx: &'a mut Transaction<'t>, table: QualifiedTableRef<'_>, key_col: &str, ) -> Self

NOTE: tx is assumed to use a utf8mb4 connection character set (the driver’s handshake default), so key values arrive converted to UTF-8. tx should be REPEATABLE READ so sequential probes see one snapshot of the table.

pub async fn estimate_range_rows( &mut self, lower_bound_exclusive: &str, upper_bound_exclusive: Option<&str>, ) -> Result<u64, MySqlError>

Estimates the row count for the given range. Estimates vary widely. On a static table with 2.2B rows we observed estimates that should be near 2B report exactly half the TABLE_ROWS reported by information_schema.tables. The sum of the row estimates from this function were around 4B for the same test case, or about a 2x overcount relative to the 2.05B reported by TABLE_ROWS reported by information_schema.tables and the 2.2B actual rows. The underlying estimates are computed by sampling a small number of pages after traversing the index (assuming this is a primary key being filtered on), so extrapolated row counts can be inaccurate but appear to eventually converge towards more accurate estimates as the sampled range shrinks on a static table.

pub async fn prefix_of_first_key_in_range( &mut self, lower_bound_exclusive: &str, upper_bound_exclusive: Option<&str>, max_prefix_length: usize, ) -> Result<Option<String>, MySqlError>

Grabs a prefix of up to max_prefix_length characters for the first key in the given range. If the key is shorter than max_prefix_length, it returns that shorter value.

The query will generally look something like:

SELECT LEFT(pk_col, 3) FROM table
WHERE pk_col > 'ab' AND pk_col < RPAD('ac', 768, CHAR(0))
ORDER BY pk_col
LIMIT 1

pub async fn prefix_of_first_row_not_matching_prefix( &mut self, prefix: &str, upper_bound_exclusive: Option<&str>, max_prefix_length: usize, ) -> Result<Option<String>, MySqlError>

Returns the prefix of up to max_prefix_length characters of the first key after prefix, but below upper_bound_exclusive. Returns None if no key matching these conditions exists.

Note: this should be run inside a REPEATABLE READ transaction because it issues two queries sequentially.