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//! Query explanation and profiling for SDBQL executor.
//!
//! This module contains the EXPLAIN functionality:
//! - explain: Generate query execution plan with timing information
use std::collections::HashMap;
use std::time::Instant;
use super::format_expression;
use super::types::{
CollectionAccess, Context, ExecutionTiming, FilterInfo, LetBinding, LimitInfo, QueryExplain,
SortInfo,
};
use super::QueryExecutor;
use crate::error::DbResult;
use crate::sdbql::ast::*;
impl<'a> QueryExecutor<'a> {
pub fn explain(&self, query: &Query) -> DbResult<QueryExplain> {
let total_start = Instant::now();
let warnings: Vec<String> = Vec::new();
let mut collections_info: Vec<CollectionAccess> = Vec::new();
let mut let_bindings_info: Vec<LetBinding> = Vec::new();
let mut filters_info: Vec<FilterInfo> = Vec::new();
// Timing accumulators
let mut collection_scan_us: u64 = 0;
let mut filter_us: u64 = 0;
let mut sort_us: u64 = 0;
let mut limit_us: u64 = 0;
let mut return_projection_us: u64 = 0;
// First, evaluate all LET clauses
let let_start = Instant::now();
let mut initial_bindings: Context = HashMap::new();
for (key, value) in &self.bind_vars {
initial_bindings.insert(format!("@{}", key), value.clone());
}
for let_clause in &query.let_clauses {
let clause_start = Instant::now();
let is_subquery = matches!(let_clause.expression, Expression::Subquery(_));
let value =
self.evaluate_expr_with_context(&let_clause.expression, &initial_bindings)?;
initial_bindings.insert(let_clause.variable.clone(), value);
let clause_time = clause_start.elapsed();
let_bindings_info.push(LetBinding {
variable: let_clause.variable.clone(),
is_subquery,
time_us: clause_time.as_micros() as u64,
});
}
let let_clauses_time = let_start.elapsed();
let mut let_clauses_us = let_clauses_time.as_micros() as u64;
// Execution Phase - Measure everything
let mut total_docs_scanned = 0usize;
let mut rows: Vec<Context> = vec![initial_bindings.clone()];
// Optimization: Check if we can push LIMIT down to storage scan
let scan_limit = if query.sort_clause.is_none() {
let for_count = query
.body_clauses
.iter()
.filter(|c| matches!(c, BodyClause::For(_)))
.count();
let filter_count = query
.body_clauses
.iter()
.filter(|c| matches!(c, BodyClause::Filter(_)))
.count();
if for_count == 1 && filter_count == 0 {
query.limit_clause.as_ref().and_then(|l| {
let (offset, count) = self.eval_limit(l, &initial_bindings);
// No count means no upper bound: nothing to push down.
offset.checked_add(count?)
})
} else {
None
}
} else {
None
};
// Iterate through body clauses (FOR, FILTER, etc.)
let clauses = if !query.body_clauses.is_empty() {
&query.body_clauses
} else {
// Fallback for empty body clauses (legacy path not fully instrumented here)
&query.body_clauses
};
let mut i = 0;
while i < clauses.len() {
match &clauses[i] {
BodyClause::For(for_clause) => {
let scan_start = Instant::now();
// Optimization: Check for Index Usage (Index Scan)
let mut used_index = false;
let mut index_name: Option<String> = None;
let mut index_type: Option<String> = None;
let mut auto_index_candidate: Option<String> = None;
// Check if next clause is a FILTER that can use an index
if i + 1 < clauses.len() {
if let BodyClause::Filter(filter_clause) = &clauses[i + 1] {
let is_collection = if let Some(src) = &for_clause.source_variable {
src == &for_clause.collection
} else {
!for_clause.collection.is_empty()
};
if is_collection {
if let Ok(collection) = self.get_collection(&for_clause.collection)
{
// EXPLAIN uses the first row's context (or
// an empty one) — sufficient for the
// structural decision of whether an index
// would be used at runtime.
let probe_ctx = rows.first().cloned().unwrap_or_default();
if let Some((docs, name, ty)) = self.lookup_index_for_filter(
&collection,
&filter_clause.expression,
&for_clause.variable,
&probe_ctx,
) {
used_index = true;
index_name = Some(name);
index_type = Some(ty);
let mut new_rows = Vec::new();
for ctx in &rows {
for doc in &docs {
let mut new_ctx = ctx.clone();
new_ctx.insert(
for_clause.variable.clone(),
doc.to_value(),
);
new_rows.push(new_ctx);
}
}
rows = new_rows;
total_docs_scanned += docs.len();
i += 2; // Skip FOR and FILTER
} else if let Some(cond) = self.extract_indexable_condition(
&filter_clause.expression,
&for_clause.variable,
&probe_ctx,
) {
if self.would_auto_index(
&collection,
&cond.field,
Some(&cond.value),
) {
auto_index_candidate = Some(cond.field);
}
}
}
}
}
}
if !used_index {
// Full Scan or Range
let mut new_rows = Vec::new();
let mut clause_docs_scanned = 0;
for ctx in &rows {
// Measure iterator creation/fetching time as part of scan
let docs = self.get_for_source_docs(for_clause, ctx, scan_limit)?;
clause_docs_scanned += docs.len();
for doc in docs {
let mut new_ctx = ctx.clone();
new_ctx.insert(for_clause.variable.clone(), doc);
new_rows.push(new_ctx);
}
}
rows = new_rows;
total_docs_scanned += clause_docs_scanned;
i += 1;
}
collection_scan_us += scan_start.elapsed().as_micros() as u64;
// Record Collection Info
collections_info.push(CollectionAccess {
name: for_clause.collection.clone(),
variable: for_clause.variable.clone(),
access_type: if used_index {
"index_lookup".to_string()
} else if scan_limit.is_some() {
"limited_scan".to_string()
} else {
"full_scan".to_string()
},
index_used: index_name,
index_type,
documents_count: 0, // Simplified
auto_index_candidate,
});
}
BodyClause::Filter(filter_clause) => {
let filter_start = Instant::now();
let before_count = rows.len();
rows.retain(|ctx| {
self.evaluate_filter_with_context(&filter_clause.expression, ctx)
.unwrap_or(false)
});
let after_count = rows.len();
let duration = filter_start.elapsed().as_micros() as u64;
filter_us += duration;
filters_info.push(FilterInfo {
expression: format_expression(&filter_clause.expression),
index_candidate: None,
can_use_index: false, // Already checked in FOR loop optimization
documents_before: before_count,
documents_after: after_count,
time_us: duration,
});
i += 1;
}
BodyClause::Let(let_clause) => {
let let_start = Instant::now();
for ctx in &mut rows {
let value = self.evaluate_expr_with_context(&let_clause.expression, ctx)?;
ctx.insert(let_clause.variable.clone(), value);
}
let_clauses_us += let_start.elapsed().as_micros() as u64;
i += 1;
}
BodyClause::Insert(_) | BodyClause::Update(_) | BodyClause::Remove(_) => {
i += 1;
}
_ => {
i += 1;
}
}
}
// Apply SORT
if let Some(sort) = &query.sort_clause {
let sort_start = Instant::now();
rows = self.sort_rows(rows, &sort.fields);
sort_us = sort_start.elapsed().as_micros() as u64;
}
// Apply LIMIT
let mut documents_returned = rows.len();
let mut limit_offset_val: usize = 0;
let mut limit_count_val: Option<usize> = None;
if let Some(limit) = &query.limit_clause {
let limit_start = Instant::now();
let (offset, count) = self.eval_limit(limit, &initial_bindings);
limit_offset_val = offset;
limit_count_val = count;
let start = limit_offset_val.min(rows.len());
let end = match limit_count_val {
Some(count) => start.saturating_add(count).min(rows.len()),
None => rows.len(),
};
rows = rows[start..end].to_vec();
documents_returned = rows.len();
limit_us = limit_start.elapsed().as_micros() as u64;
}
// Apply RETURN Projection
if let Some(ref return_clause) = query.return_clause {
let proj_start = Instant::now();
for ctx in &rows {
let _ = self.evaluate_expr_with_context(&return_clause.expression, ctx);
}
return_projection_us = proj_start.elapsed().as_micros() as u64;
}
let total_us = total_start.elapsed().as_micros() as u64;
Ok(QueryExplain {
collections: collections_info,
let_bindings: let_bindings_info,
filters: filters_info,
sort: query.sort_clause.as_ref().map(|s| SortInfo {
field: s
.fields
.iter()
.map(|(e, _)| format_expression(e))
.collect::<Vec<_>>()
.join(", "),
direction: if s.fields.first().map(|(_, asc)| *asc).unwrap_or(true) {
"ASC".to_string()
} else {
"DESC".to_string()
},
time_us: sort_us,
}),
limit: query.limit_clause.as_ref().map(|_l| LimitInfo {
offset: limit_offset_val,
count: limit_count_val,
}),
timing: ExecutionTiming {
total_us,
let_clauses_us,
collection_scan_us,
filter_us,
sort_us,
limit_us,
return_projection_us,
},
documents_scanned: total_docs_scanned,
documents_returned,
warnings,
})
}
}