Merge pull request #289 from github/concurrent-rowcount-defaults-true

concurrent-rowcount defaults 'true'

Author: Shlomi Noach

doc/command-line-flags.md

@@ -71,7 +71,8 @@ A `gh-ost` execution need to copy whatever rows you have in your existing table
 `gh-ost` also supports the `--exact-rowcount` flag. When this flag is given, two things happen:
 - An initial, authoritative `select count(*) from your_table`.
   This query may take a long time to complete, but is performed before we begin the massive operations.
-  When `--concurrent-rowcount` is also specified, this runs in paralell to row copy.
+  When `--concurrent-rowcount` is also specified, this runs in parallel to row copy.
+  Note: `--concurrent-rowcount` now defaults to `true`.
 - A continuous update to the estimate as we make progress applying events.
   We heuristically update the number of rows based on the queries we process from the binlogs.
 

go/cmd/gh-ost/main.go

@@ -54,7 +54,7 @@ func main() {
 	flag.StringVar(&migrationContext.OriginalTableName, "table", "", "table name (mandatory)")
 	flag.StringVar(&migrationContext.AlterStatement, "alter", "", "alter statement (mandatory)")
 	flag.BoolVar(&migrationContext.CountTableRows, "exact-rowcount", false, "actually count table rows as opposed to estimate them (results in more accurate progress estimation)")
-	flag.BoolVar(&migrationContext.ConcurrentCountTableRows, "concurrent-rowcount", false, "(with --exact-rowcount), when true: count rows after row-copy begins, concurrently, and adjust row estimate later on; defaults false: first count rows, then start row copy")
+	flag.BoolVar(&migrationContext.ConcurrentCountTableRows, "concurrent-rowcount", true, "(with --exact-rowcount), when true (default): count rows after row-copy begins, concurrently, and adjust row estimate later on; when false: first count rows, then start row copy")
 	flag.BoolVar(&migrationContext.AllowedRunningOnMaster, "allow-on-master", false, "allow this migration to run directly on master. Preferably it would run on a replica")
 	flag.BoolVar(&migrationContext.AllowedMasterMaster, "allow-master-master", false, "explicitly allow running in a master-master setup")
 	flag.BoolVar(&migrationContext.NullableUniqueKeyAllowed, "allow-nullable-unique-key", false, "allow gh-ost to migrate based on a unique key with nullable columns. As long as no NULL values exist, this should be OK. If NULL values exist in chosen key, data may be corrupted. Use at your own risk!")