variable

const (
	SQLModeVar          = "sql_mode"
	AutocommitVar       = "autocommit"
	CharacterSetResults = "character_set_results"
	MaxAllowedPacket    = "max_allowed_packet"
	TimeZone            = "time_zone"
)

const (
	CodeUnknownStatusVar terror.ErrCode = 1
	CodeUnknownSystemVar terror.ErrCode = 1193
	CodeIncorrectScope   terror.ErrCode = 1238
)

const (
	// CollationConnection is the name for collation_connection system variable.
	CollationConnection = "collation_connection"
	// CharsetDatabase is the name for character_set_database system variable.
	CharsetDatabase = "character_set_database"
	// CollationDatabase is the name for collation_database system variable.
	CollationDatabase = "collation_database"
)

const (

	// tidb_snapshot is used for reading history data, the default value is empty string.
	// When the value is set to a datetime string like '2017-11-11 20:20:20', the session reads history data of that time.
	TiDBSnapshot = "tidb_snapshot"

	// tidb_skip_constraint_check is used for loading data from a dump file, to speed up the loading process.
	// When the value is set to true, unique index constraint is not checked.
	TiDBSkipConstraintCheck = "tidb_skip_constraint_check"

	// tidb_opt_agg_push_down is used to endable/disable the optimizer rule of aggregation push down.
	TiDBOptAggPushDown = "tidb_opt_agg_push_down"

	// tidb_opt_insubquery_unfold is used to enable/disable the optimizer rule of in subquery unfold.
	TiDBOptInSubqUnFolding = "tidb_opt_insubquery_unfold"

	// tidb_build_stats_concurrency is used to speed up the ANALYZE statement, when a table has multiple indices,
	// those indices can be scanned concurrently, with the cost of higher system performance impact.
	TiDBBuildStatsConcurrency = "tidb_build_stats_concurrency"

	// tidb_distsql_scan_concurrency is used to set the concurrency of a distsql scan task.
	// A distsql scan task can be a table scan or a index scan, which may be distributed to many TiKV nodes.
	// Higher concurrency may reduce latency, but with the cost of higher memory usage and system performance impact.
	// If the query has a LIMIT clause, high concurrency makes the system do much more work than needed.
	TiDBDistSQLScanConcurrency = "tidb_distsql_scan_concurrency"

	// tidb_index_lookup_size is used for index lookup executor.
	// The index lookup executor first scan a batch of handles from a index, then use those handles to lookup the table
	// rows, this value controls how much of handles in a batch to do a lookup task.
	// Small value sends more RPCs to TiKV, consume more system resource.
	// Large value may do more work than needed if the query has a limit.
	TiDBIndexLookupSize = "tidb_index_lookup_size"

	// tidb_index_lookup_concurrency is used for index lookup executor.
	// A lookup task may have 'tidb_index_lookup_size' of handles at maximun, the handles may be distributed
	// in many TiKV nodes, we executes multiple concurrent index lookup tasks concurrently to reduce the time
	// waiting for a task to finish.
	// Set this value higher may reduce the latency but consumes more system resource.
	TiDBIndexLookupConcurrency = "tidb_index_lookup_concurrency"

	// tidb_index_serial_scan_concurrency is used for controling the concurrency of index scan operation
	// when we need to keep the data output order the same as the order of index data.
	TiDBIndexSerialScanConcurrency = "tidb_index_serial_scan_concurrency"

	// tidb_skip_ddl_wait skips the wait tiem of two lease after executing CREATE TABLE statement.
	// When we have multiple TiDB servers in a cluster, the newly created table may not be available on all TiDB server
	// until two lease time later, set this value to true will reduce the time to create a table, with the risk that
	// other TiDB servers may fail to use the newly created table in a small time window.
	TiDBSkipDDLWait = "tidb_skip_ddl_wait"

	// tidb_skip_utf8_check skips the UTF8 validate process, validate UTF8 has performance cost, if we can make sure
	// the input string values are valid, we can skip the check.
	TiDBSkipUTF8Check = "tidb_skip_utf8_check"

	// tidb_batch_insert is used to enable/disable auto-split insert data. If set this option on, insert executor will automatically
	// insert data into multiple batches and use a single txn for each batch. This will be helpful when inserting large data.
	TiDBBatchInsert = "tidb_batch_insert"
)

const (
	DefIndexLookupConcurrency     = 4
	DefIndexSerialScanConcurrency = 1
	DefIndexLookupSize            = 20000
	DefDistSQLScanConcurrency     = 10
	DefBuildStatsConcurrency      = 4
	DefSkipDDLWait                = false
	DefSkipUTF8Check              = false
	DefOptAggPushDown             = true
	DefOptInSubqUnfolding         = false
	DefBatchInsert                = false
)