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InnoDB is MySQL's default storage engine, providing ACID transactions, row-level locking, and a buffer pool for caching data and indexes.
No evidence-backed relationships are recorded.
Postgres locks both tables with SHARE ROW EXCLUSIVE when a migration adds a foreign key, blocking writes until every child row is scanned. A dev.to guide on Laravel migrations splits the change into two statements so the slow scan stops holding up the app's writes.
Reality
A dev.to walkthrough of PostgreSQL snapshots, LSNs and Oracle SCNs sets out why a row version has no commit coordinate to record when it is written, and what that indirection costs the replication and audit consumers that read it later.
Reality
InnoDB applies MVCC to reads and pure locking to writes, so under the default Repeatable Read the same predicate can be answered from a snapshot frozen at your first statement or from the newest committed row.
Reality
Mercari's DBRE team found that on tables with a composite or non-integer primary key, TiDB reads every matching index entry and then sorts. The upstream fix is in master, not in the current LTS.
Reality
Prices in Magento 2 are computed from tier prices, catalog rules, groups and staging, not read from a column. That is why the price indexer scales worst, and why hand-written SQL fixes backfire.
Reality
A walkthrough of OrioleDB's concurrency model puts the cost of Postgres MVCC in the search-key space. That is also the reason zheap's table-only fix was never enough.
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A dev.to write-up argues Multi-Source Inventory leaves an unbounded reservation table in the read path of every cart operation. The reported cost: 200-800ms per product, per add-to-cart.
Reality