Class: TMMUpdater
function Prefetch(const AWhere: string): Boolean;Loads rows from the remote MySQL/MariaDB table into the local EDB
buffer, using the same SELECT script pipeline as TMMScript.
The buffer is cleared before loading. Returns True when the
server responded successfully, including when zero rows matched.
Prepare must have been called first. Only available in
umUpdate and umUpsert modes.
AWhere is a plain SQL WHERE fragment applied to the
SELECT, e.g.
'status = ''pending'' AND region = ''EU'''.
Pass an empty string to load all rows.
BLOB and CLOB columns: Prefetch always
downloads BLOB and CLOB values in full — there is no size cap on what
goes into the buffer. This is intentional: every column fetched by
Prefetch will be written back to the server by
Execute. A truncated value in the buffer would be written
back as a truncated value, permanently overwriting the original
data.
When a BLOB or CLOB cell was stored as a server-side temp file
(because it exceeded the out-of-band threshold), Prefetch
automatically fetches the full value and places it in the buffer. This
repair step requires the table to have a primary key. If any cell cannot
be fully retrieved, Prefetch returns False and
Execute is blocked until Prefetch is called
again successfully.
If the table contains large BLOB or CLOB columns that do not need to
be updated, exclude them from UpdateColumns.
They will then be absent from the buffer, absent from
Prefetch, and absent from Execute.
// Update status and timestamp only; leave 'image' and 'document' untouched
Updater.UpdateColumns := ['id', 'status', 'updated_at'];
Updater.Prepare;
Updater.Prefetch('status = ''pending''');