In real-world tasks like data management and Web search, index operations often exhibit strong skewness, unlike standard benchmarks with uniform data distribution. While learned indexes improve query and update efficiency, they typically fail to address the skewed workload access, often prioritizing a single performance metric at the cost of overall index effectiveness. Additionally, the full reliance on learned models can increase vulnerability to attacks, compromising system stability. To address these challenges, we propose ShapeShifter, an adaptive evolutionary structure based on traditional indexes, capable of dynamically adjusting node structures according to the workload. ShapeShifter introduces a node evolution strategy with workload-skew-aware policies to adaptively adjust and optimize the partial index structure, leveraging a hybrid mechanism that combines traditional and learned structures for robust performance and optimal time-space tradeoff under skewed workloads and extreme data conditions. The evaluation results show that ShapeShifter achieves the optimal tradeoff while maintaining robustness.

The first generation of learned indexes inherently achieved lower space overhead than traditional index structures, establishing this advantage as one of the pivotal research directions in index optimization. However, in their pursuit of peak performance, designers often significantly increase space overhead, which becomes infeasible in scenarios with limited storage space. Furthermore, the design of current learned indexes optimized for time-space tradeoff is flawed, as they collapse catastrophically under prevalent dense or duplicate insertion workloads. To address these challenges, we first quantitatively analyze the time-space correlation characteristics of learned indexes from a theoretical perspective and identify the core influencing factors. Based on this, time-space cost minimization function models are established and an updatable learned index framework, LIFT, is constructed. Furthermore, LIFT incorporates specifically designed structural adjustment mechanisms to effectively counter existing poisoning attacks, significantly enhancing index robustness without increasing time-space cost. Evaluation results demonstrate that LIFT consistently achieves the optimal time-space tradeoff across various workloads and datasets, outperforming all other state-of-the-art indexes.