GBase 8a Performance Anomaly Case Study: How a Single Parameter Change Sparked a Chain Reaction

Short summary

A single parameter adjustment (group_concat_max_len: 32KB → 1MB) triggered a hidden cascade in GBase 8a: the optimizer inferred LONGTEXT columns, pre-allocating 64MB per row for a 200k-row sort, spilling ~12TB to disk and starving the node with writes. Fixes: wrap group_concat() in substr() to control column width, or use statement-level hints to isolate parameter changes from other workloads.

• •Parameter change (group_concat_max_len 32KB→1MB) caused optimizer to infer LONGTEXT instead of VARCHAR
• •Resulted in 64MB per-row memory pre-allocation; 200k rows = 12TB+ disk spill and multi-hour query stalls
• •Solutions: wrap group_concat() with substr() to cap width, or use statement-level hints to isolate parameters

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