The constant flux of data and queries alike has been pushing the boundaries of data analysis systems. The increasing size of raw data files has made data loading an expensive operation that delays the data-to-insight time. To alleviate the loading cost, in situ query processing systems operate directly over raw data and offer instant access to data. At the same time, analytical workloads have increasing number of queries. Typically, each query focuses on a constantly shifting-yet small-range. As a result, minimizing the workload latency requires the benefits of indexing in in situ query processing. In this paper, we present an online partitioning and indexing scheme, along with a partitioning and indexing tuner tailored for in situ querying engines. The proposed system design improves query execution time by taking into account user query patterns, to (i) partition raw data files logically and (ii) build lightweight partition-specific indexes for each partition. We build an in situ query engine called Slalom to showcase the impact of our design. Slalom employs adaptive partitioning and builds non-obtrusive indexes in different partitions on-the-fly based on lightweight query access pattern monitoring. As a result of its lightweight nature, Slalom achieves efficient query processing over raw data with minimal memory consumption. Our experimentation with both microbenchmarks and real-life workloads shows that Slalom outperforms state-of-the-art in situ engines and achieves comparable query response times with fully indexed DBMS, offering lower cumulative query execution times for query workloads with increasing size and unpredictable access patterns.