15.15.5.1. Implementing the info() Function
The optimizer requests an update of table information by
calling the
handler::info()
function. The info() function does not
have a return value, instead it is expected that the storage
engine will set a variety of public variables that the server
will then read as needed. These values are also used to
populate certain SHOW outputs such as
SHOW TABLE STATUS and for queries of the
INFORMATION_SCHEMA.
All variables are optional but should be filled if possible:
records - The number of rows in the
table. If you cannot provide an accurate number quickly
you should set the value to be greater than 1 so that the
optimizer does not perform optimizations for zero or one
row tables.
deleted - Number of deleted rows in
table. Used to identify table fragmentation, where
applicable.
data_file_length - Size of the data
file, in bytes. Helps optimizer calculate the cost of
reads.
index_file_length - Size of the index
file, in bytes. Helps optimizer calculate the cost of
reads.
mean_rec_length - Average length of a
single row, in bytes.
scan_time - Cost in I/O seeks to
perform a full table scan.
delete_length -
check_time -
When calculating values, speed is more important than
accuracy, as there is no sense in taking a long time to give
the optimizer clues as to what approach may be the fastest.
Estimates within an order of magnitude are usually good
enough.
