Ported from openGauss annkmeans.cpp:setupKmeansState. For PQ training, cosine/inner-product collapse to L2_SQRT on (already-or-implicitly) normalized

Add chunk to freelist */

Try to allocate from global freelist first */

Allocate a new block from ShmemAlloc */

* Get the approximate number of elements in the filter */

逗号分隔参数, 但忽略 [] / () / '' 内的逗号.

剥离 ${VEC_LITERAL([(expr)::FLOAT, ...], 4)} / ${L2(a, b)} / ${SYS_INDEXES}. 平衡扫描支持任意嵌套 () [] 和嵌套 ${...}, 替换为 @T@. 旧实现用 `\\([^)]*\\)` 在第一个 `)`

用 shared counter 验证 reader/writer 互斥

Verify LWLockConditionalAcquire really is non-blocking.

Reserve and rehash

\copydoc boost::lockfree::queue::unsynchronized_push(const T& t)

Cache invalidation */

Calculate shared memory size needed */

* Initialize GUC parameters and reloptions */

Welch's t-test, two-sided. a = candidate, b = baseline.