divine-mobile: Skill for Claude Code

ClickHouse Rust: View Computed Column Ordering

Problem

When using SELECT alias.* , extra_col FROM my_view alias LEFT JOIN ..., ClickHouse expands alias.* to include ALL columns defined in the view — including any computed columns added by the view itself (e.g. SELECT *, expr AS trending_score FROM base). Those computed columns appear INSIDE the alias.* expansion, BEFORE any columns appended after it in the outer query.

The clickhouse Rust crate (#[derive(Row)]) deserializes positionally — field N in the Rust struct receives column N from the query result. If the struct field order doesn't match the actual SQL column order, the wrong bytes land in the wrong fields, causing type errors at deserialization even though the column count is correct.

Symptoms

• HTTP 500 on one sort variant (e.g. sort=trending, sort=popular) but not others (e.g. sort=recent) that query a different view or table
• Error like: "cannot decode Float64 from String" or similar type mismatch
• Column COUNT is correct (no "not enough data" error)
• Bug appears after adding a computed column to an existing view

Root Cause Explained

Suppose trending_videos is defined as:

CREATE VIEW trending_videos AS
SELECT
    *,                                          -- all base columns
    (views * 0.5 + likes * 2.0) AS trending_score  -- computed column APPENDED HERE
FROM video_stats;

An outer query then does:

SELECT
    tv.*,                   -- expands to: base_cols..., trending_score
    text_track_ref,         -- subtitle columns come AFTER
    text_track_content
FROM trending_videos tv
LEFT JOIN subtitle_subquery USING (id);

The actual SQL result column order is:

[...base_cols, trending_score, text_track_ref, text_track_content]

But if the Rust struct was written with subtitle fields before trending_score:

pub struct TrendingVideo {
    // ...base fields...
    pub text_track_ref: String,      // position N   <- WRONG: gets trending_score bytes
    pub text_track_content: String,  // position N+1 <- WRONG: gets text_track_ref bytes
    pub trending_score: f64,         // position N+2 <- WRONG: gets text_track_content bytes
}