performance - MySQL updates getting very slow towards end of the table -


i have table "data" holds around 100,000,000 records. have added new column "batch_id" (integer).

on application layer, i'm updating batch_id in batches of 10,000 records each of 100,000,000 records (the batch_id same 10k).

i'm doing (application layer pseudo code):

loop {   $batch_id = $batch_id + 1;   mysql.query("update data set batch_id='$batch_id' batch_id null limit 10000"); }

i have index on batch_id column.

in beginning, update statement took ~30 seconds. i'm halfway through table , it's getting slower , slower. @ moment same statement takes around 10 minutes(!). reached point no longer feasible take on month update whole table @ current speed.

what speed up, , why mysql getting slower towards end of table? index on primary key help?

is primary key automatically indexed in mysql? answer yes

so instead 1 index batch_id help.

the problem without index engine full table scan. @ first easy find 10k null values, when more , more records updated engine have scan more find nulls.

but should easier create batch_id autonumeric column

other option: create new table , add index , replace old table.

create newtable      select if(@newid := @newid + 1,               @newid div 10000,               @newid div 10000) batch_id,            <other fields>     yourtable              cross join (select @newid :=0 ) v

insert auto increment primary key existing table


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