This could be one of the following (to start with):

• linear, e.g. A ⋈ B, B ⋈ C, C ⋈ D
• star, e.g. A ⋈ B, A ⋈ C, A ⋈ D
• fully connected, e.g. A ⋈ B, A ⋈ C, A ⋈ D,  B ⋈ C, C ⋈ D,  B ⋈ D
• unknown

... and add it to the JoinOptimization info query stats type

We should also include information about presence/absence of cycles here.