Consider this happening in the 1st second of 10:15 am
@10:15:01:025 User 1 inserts to Table 1
@10:15:01:050 User 1 Selects last inserted pkey from Table 1
@10:15:01:075 User 1 inserts to Table 2
All good
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Now consider this alternate scenario happening in the same 1st second of 10:15 am
@10:15:01:025 User 1 inserts to Table 1
@10:15:01:030 User 2 inserts to Table 1
@10:15:01:050 User 1 Selects last inserted pkey from Table 1
@10:15:01:075 User 1 inserts to Table 2
Chaos! All in less than a second