also passes. Finally, TiDB has a theoretical dependence on CLOCK_MONOTONIC_RAW for safety, but we have not yet observed anomalies due to clock skew, possibly due to tooling limitations. PingCAP has published are some wall-clock dependencies in TiDB. Placement Driver, for instance, allocates timestamps based on local wall-clocks, but ensures monotonicity through Raft plus a leader lease. Clock skew between PD shouldn’t result in prolonged un- availability: PD will allocate timestamps in the future if its own clock falls behind what’s committed to Raft. To improve throughput, PD nodes reserve a batch of timestamps

Types CRDT – Last Write Wins T2 > T1 T2 WINS 时间不一致 两个Redis的系 统时钟不一致 最终保留了第一 次的结果 Vector Clock • 向量表示不同节点 的操作数 • SET KEY=VAL1 (0,0)->（1,0） • SET KEY=VAL2 （1,0）->（1,1） 删除导致数据不一致 • 假设已经存在 保留被删除的记录 内存爆满 随着时间的推移，大量的失效KEY驻留 时间轴 CRDT -- GC 基于节点之间的vector clock的通讯，删除不必要的 失效KEY （1,0） （1,1） （1,1） Vector Clock Vector Clock （1,0） 什么是 CRDT State-based Replication • 交换律 • 结合律 • 幂等性 Op-based

12-hour clock (1-12). hh Display the hour as a number with a leading zero using the 12-hour clock (01-12). H Display the hour as a number without a leading zero using the 24-hour clock (0-23) (0-23). HH Display the hour as a number with a leading zero using the 24-hour clock (00-23). m Display the minute as a number without a leading zero (0-59). mm Display the minute as a number with 12-hour clock (1-12). hh Display the hour as a number with a leading zero using the 12-hour clock (01-12). H Display the hour as a number without a leading zero using the 24-hour clock (0-23)

12-hour clock (1-12). hh Display the hour as a number with a leading zero using the 12-hour clock (01-12). H Display the hour as a number without a leading zero using the 24-hour clock (0-23) (0-23). HH Display the hour as a number with a leading zero using the 24-hour clock (00-23). m Display the minute as a number without a leading zero (0-59). mm Display the minute as a number with

12-hour clock (1-12). hh Display the hour as a number with a leading zero using the 12-hour clock (01-12). H Display the hour as a number without a leading zero using the 24-hour clock (0-23) (0-23). HH Display the hour as a number with a leading zero using the 24-hour clock (00-23). m Display the minute as a number without a leading zero (0-59). mm Display the minute as a number with

12-hour clock (1-12). hh Display the hour as a number with a leading zero using the 12-hour clock (01-12). H Display the hour as a number without a leading zero using the 24-hour clock (0-23) (0-23). HH Display the hour as a number with a leading zero using the 24-hour clock (00-23). m Display the minute as a number without a leading zero (0-59). mm Display the minute as a number with

252 clock (1-12). hh Display the hour as a number with a leading zero using the 12-hour clock (01-12). H Display the hour as a number without a leading zero using the 24-hour clock (0-23) (0-23). HH Display the hour as a number with a leading zero using the 24-hour clock (00-23). m Display the minute as a number without a leading zero (0-59). mm Display the minute as a number with Display the time using the format given by the system global variable. am/pm Use the 12-hour clock for the preceding h or hh specifier, and display 'am' for any hour before noon, and 'pm' for any

extract(dow from ...)’s day number- ing. • to_char(interval) formats HH and HH12 as shown on a 12-hour clock, i.e. zero hours and 36 hours output as 12, while HH24 outputs the full hour value, which can exceed interval Subtract from current_date (at midnight) age(timestamp ’1957-06-13’) 43 years 8 mons 3 days clock_timestamp() timestamp with time zone Current date and time (changes during statement execution); time (start of current statement); see Section 9.9.4 timeofday() text Current date and time (like clock_timestamp, but as a text string); see Section 9.9.4 transaction_timestamp() timestamp with time

extract(dow from ...)’s day numbering. • to_char(interval) formats HH and HH12 as shown on a 12-hour clock, i.e. zero hours and 36 hours output as 12, while HH24 outputs the full hour value, which can exceed interval Subtract from current_date (at midnight) age(timestamp ’1957-06-13’) 43 years 8 mons 3 days clock_timestamp() timestamp with time zone Current date and time (changes during statement execution); time (start of current statement); see Section 9.9.4 timeofday() text Current date and time (like clock_timestamp, but as a text string); see Section 9.9.4 transaction_timestamp() timestamp with time

extract(dow from ...)’s day number- ing. • to_char(interval) formats HH and HH12 as shown on a 12-hour clock, i.e. zero hours and 36 hours output as 12, while HH24 outputs the full hour value, which can exceed interval Subtract from current_date (at midnight) age(timestamp ’1957-06-13’) 43 years 8 mons 3 days clock_timestamp() timestamp with time zone Current date and time (changes during statement execution); time (start of current statement); see Section 9.9.4 timeofday() text Current date and time (like clock_timestamp, but as a text string); see Section 9.9.4 211 Chapter 9. Functions and Operators Function