diff options
Diffstat (limited to 'src/resolve.c')
| -rw-r--r-- | src/resolve.c | 21 |
1 files changed, 12 insertions, 9 deletions
diff --git a/src/resolve.c b/src/resolve.c index 668498cb9..32914befb 100644 --- a/src/resolve.c +++ b/src/resolve.c @@ -788,16 +788,19 @@ static int resolveExprStep(Walker *pWalker, Expr *pExpr){ break; } - /* An "<expr> IS NOT NULL" or "<expr> IS NULL". After resolving the - ** LHS, check if there is a NOT NULL constraint in the schema that - ** means the value of the expression can be determined immediately. - ** If that is the case, replace the current expression node with - ** a TK_TRUEFALSE node. + /* An optimization: Attempt to convert ** - ** If the node is replaced with a TK_TRUEFALSE node, then also restore - ** the NameContext ref-counts to the state they where in before the - ** LHS expression was resolved. This prevents the current select - ** from being erroneously marked as correlated in some cases. + ** "expr IS NOT NULL" --> "TRUE" + ** "expr IS NULL" --> "FALSE" + ** + ** if we can prove that "expr" is never NULL. Call this the + ** "NOT NULL strength reduction optimization". + ** + ** If this optimization occurs, also restore the NameContext ref-counts + ** to the state they where in before the "column" LHS expression was + ** resolved. This prevents "column" from being counted as having been + ** referenced, which might prevent a SELECT from being erroneously + ** marked as correlated. */ case TK_NOTNULL: case TK_ISNULL: { |