Move where clause literal out of loop

core/translate.rs

@@ -269,6 +269,7 @@ fn translate_select(mut select: Select) -> Result<Program> {
     }
     program.resolve_label(init_label, program.offset());
     program.emit_insn(Insn::Transaction);
+    program.emit_run_once_insns();
     program.emit_insn(Insn::Goto {
         target_pc: start_offset,
     });
@@ -501,7 +502,15 @@ fn translate_condition_expr(
             let e1_reg = program.alloc_register();
             let e2_reg = program.alloc_register();
             let _ = translate_expr(program, select, e1, e1_reg)?;
+            match e1.as_ref() {
+                ast::Expr::Literal(_) => program.move_last_insn_out_of_loop(),
+                _ => {}
+            }
             let _ = translate_expr(program, select, e2, e2_reg)?;
+            match e2.as_ref() {
+                ast::Expr::Literal(_) => program.move_last_insn_out_of_loop(),
+                _ => {}
+            }
             if jump_target < 0 {
                 program.add_label_dependency(jump_target, program.offset());
             }
@@ -1030,6 +1039,7 @@ fn translate_pragma(
     program.emit_insn(Insn::Halt);
     program.resolve_label(init_label, program.offset());
     program.emit_insn(Insn::Transaction);
+    program.emit_run_once_insns();
     program.emit_insn(Insn::Goto {
         target_pc: start_offset,
     });

core/vdbe.rs

@@ -225,6 +225,8 @@ pub struct ProgramBuilder {
     next_free_label: BranchOffset,
     next_free_cursor_id: usize,
     insns: Vec<Insn>,
+    // for temporarily storing instructions that will be put after Transaction opcode
+    run_once_insns: Vec<Insn>,
     // Each label has a list of InsnReferences that must
     // be resolved. Lists are indexed by: label.abs() - 1
     unresolved_labels: Vec<Vec<InsnReference>>,
@@ -243,6 +245,7 @@ impl ProgramBuilder {
             unresolved_labels: Vec::new(),
             next_insn_label: None,
             cursor_ref: Vec::new(),
+            run_once_insns: Vec::new(),
         }
     }
 
@@ -281,6 +284,18 @@ impl ProgramBuilder {
         }
     }
 
+    // Emit an instruction that will be put at the end of the program (after Transaction statement).
+    // This is useful for instructions that otherwise will be unnecessarily repeated in a loop.
+    // Example: In `SELECT * from users where name='John'`, it is unnecessary to set r[1]='John' as we SCAN users table.
+    // We could simply set it once before the SCAN started.
+    pub fn move_last_insn_out_of_loop(&mut self) {
+        self.run_once_insns.push(self.insns.pop().unwrap());
+    }
+
+    pub fn emit_run_once_insns(&mut self) {
+        self.insns.extend(self.run_once_insns.drain(..));
+    }
+
     pub fn emit_insn_with_label_dependency(&mut self, insn: Insn, label: BranchOffset) {
         self.insns.push(insn);
         self.add_label_dependency(label, (self.insns.len() - 1) as BranchOffset);
@@ -430,6 +445,10 @@ impl ProgramBuilder {
     }
 
     pub fn build(self) -> Program {
+        assert!(
+            self.run_once_insns.is_empty(),
+            "run_once_insns is not empty when build() is called, did you forget to call emit_run_once_insns()?"
+        );
         Program {
             max_registers: self.next_free_register,
             insns: self.insns,