Scalar Filtering Rules

Overview

A predicate expression outputs a boolean value. Milvus conducts scalar filtering by searching with predicates. A predicate expression, when evaluated, returns either TRUE or FALSE. View Python SDK API Reference for instruction on using predicate expressions.

EBNF grammar rules describe boolean expressions rules:

Expr = LogicalExpr | NIL
LogicalExpr = LogicalExpr BinaryLogicalOp LogicalExpr 
              | UnaryLogicalOp LogicalExpr
              | "(" LogicalExpr ")"
              | SingleExpr;
BinaryLogicalOp = "&&" | "and" | "||" | "or";
UnaryLogicalOp = "not";
SingleExpr = TermExpr | CompareExpr;
TermExpr = IDENTIFIER "in" ConstantArray;
Constant = INTEGER | FLOAT
ConstantExpr = Constant
               | ConstantExpr BinaryArithOp ConstantExpr
               | UnaryArithOp ConstantExpr;
                                                          
ConstantArray = "[" ConstantExpr { "," ConstantExpr } "]";
UnaryArithOp = "+" | "-"
BinaryArithOp = "+" | "-" | "*" | "/" | "%" | "**";
CompareExpr = IDENTIFIER CmpOp IDENTIFIER
              | IDENTIFIER CmpOp ConstantExpr
              | ConstantExpr CmpOp IDENTIFIER
              | ConstantExpr CmpOpRestricted IDENTIFIER CmpOpRestricted ConstantExpr;
CmpOpRestricted = "<" | "<=";
CmpOp = ">" | ">=" | "<" | "<=" | "=="| "!=";
MatchOp = "like" | "LIKE";
JsonArrayOps = JsonDefs "(" IDENTIFIER "," JsonExpr | JsonArray ")";
JsonArrayDefs = "json_contains" | "JSON_CONTAINS" 
           | "json_contains_all" | "JSON_CONTAINS_ALL" 
           | "json_contains_any" | "JSON_CONTAINS_ANY";
JsonExpr =  Constant | ConstantArray | STRING | BOOLEAN;
JsonArray = "[" JsonExpr { "," JsonExpr } "]";
ArrayOps = ArrayDefs "(" IDENTIFIER "," ArrayExpr | Array ")";
ArrayDefs = "array_contains" | "ARRAY_CONTAINS" 
           | "array_contains_all" | "ARRAY_CONTAINS_ALL" 
           | "array_contains_any" | "ARRAY_CONTAINS_ANY"
           | "array_length"       | "ARRAY_LENGTH";
ArrayExpr =  Constant | ConstantArray | STRING | BOOLEAN;
Array = "[" ArrayExpr { "," ArrayExpr } "]";

The following table lists the description of each symbol mentioned in the above Boolean expression rules.

Notation	Description
=	Definition.
,	Concatenation.
;	Termination.
|	Alternation.
{…}	Repetition.
(…)	Grouping.
NIL	Empty. The expression can be an empty string.
INTEGER	Integers such as 1, 2, 3.
FLOAT	Float numbers such as 1.0, 2.0.
CONST	Integers or float numbers.
IDENTIFIER	Identifier. In Milvus, the IDENTIFIER represents the field name.
LogicalOp	A LogicalOp is a logical operator that supports combining more than one relational operation in one comparison. Returned value of a LogicalOp is either TRUE (1) or FALSE (0). There are two types of LogicalOps, including BinaryLogicalOps and UnaryLogicalOps.
UnaryLogicalOp	UnaryLogicalOp refers to the unary logical operator "not".
BinaryLogicalOp	Binary logical operators that perform actions on two operands. In a complex expression with two or more operands, the order of evaluation depends on precedence rules.
ArithmeticOp	An ArithmeticOp, namely an arithmetic operator, performs mathematical operations such as addition and subtraction on operands.
UnaryArithOp	A UnaryArithOp is an arithmetic operator that performs an operation on a single operand. The negative UnaryArithOp changes a positive expression into a negative one, or the other way round.
BinaryArithOp	A BinaryArithOp, namely a binary operator, performs operations on two operands. In a complex expression with two or more operands, the order of evaluation depends on precedence rules.
CmpOp	CmpOp is a relational operator that perform actions on two operands.
CmpOpRestricted	CmpOpRestricted is restricted to “Less than” and "Equal".
ConstantExpr	ConstantExpr can be a Constant or a BinaryArithOp on two ConstExprs or a UnaryArithOp on a single ConstantExpr. It is defined recursively.
ConstantArray	ConstantArray is wrapped by square brackets, and ConstantExpr can be repeated in the square brackets. ConstArray must include at least one ConstantExpr.
TermExpr	TermExpr is used to check whether the value of an IDENTIFIER appears in a ConstantArray. TermExpr is represented by "in".
CompareExpr	A CompareExpr, namely comparison expression can be relational operations on two IDENTIFIERs, or relational operations on one IDENTIFIER and one ConstantExpr, or ternary operation on two ConstantExprs and one IDENTIFIER.
SingleExpr	SingleExpr, namely single expression, can be either a TermExpr or a CompareExpr.
LogicalExpr	A LogicalExpr can be a BinaryLogicalOp on two LogicalExprs, or a UnaryLogicalOp on a single LogicalExpr, or a LogicalExpr grouped within parentheses, or a SingleExpr. The LogicalExpr is defined recursively.
Expr	Expr, an abbreviation meaning expression, can be LogicalExpr or NIL.
MatchOp	A MatchOp, namely a match operator, compares a string to a string constant or a string prefix, infix, or suffix constant.
JsonArrayOp	A JsonOp, namely a JSON operator, checks whether the specified identifier contains the specified elements.
ArrayOp	An ArrayOp, namely an array operator, checks whether the specified identifier contains the specified elements.

Operators

Logical operators

Logical operators perform a comparison between two expressions.

Symbol	Operation	Example	Description
‘and’ &&	and	expr1 && expr2	True if both expr1 and expr2 are true.
‘or’ ||	or	expr1 || expr2	True if either expr1 or expr2 are true.

Binary arithmetic operators

Binary arithmetic operators contain two operands and can perform basic arithmetic operations and return the corresponding result.

Symbol	Operation	Example	Description
+	Addition	a + b	Add the two operands.
-	Subtraction	a - b	Subtract the second operand from the first operand.
*	Multiplication	a * b	Multiply the two operands.
/	Division	a / b	Divide the first operand by the second operand.
**	Power	a ** b	Raise the first operand to the power of the second operand.
%	Modulo	a % b	Divide the first operand by the second operand and yield the remainder portion.

Relational operators

Relational operators use symbols to check for equality, inequality, or relative order between two expressions.

Symbol	Operation	Example	Description
<	Less than	a < b	True if a is less than b.
>	Greater than	a > b	True if a is greater than b.
==	Equal	a == b	True if a is equal to b.
!=	Not equal	a != b	True if a is not equal to b.
<=	Less than or equal	a <= b	True if a is less than or equal to b.
>=	Greater than or equal	a >= b	True if a is greater than or equal to b.

Operator precedence and associativity

The following table lists the precedence and associativity of operators. Operators are listed top to bottom, in descending precedence.

Precedence	Operator	Description	Associativity
1	+ -	UnaryArithOp	Left-to-right
2	not	UnaryLogicOp	Right-to-left
3	**	BinaryArithOp	Left-to-right
4	* / %	BinaryArithOp	Left-to-right
5	+ -	BinaryArithOp	Left-to-right
6	< <= > >=	CmpOp	Left-to-right
7	== !=	CmpOp	Left-to-right
8	like LIKE	MatchOp	Left-to-right
9	json_contains JSON_CONTAINS	JsonArrayOp	Left-to-right
9	array_contains ARRAY_CONTAINS	ArrayOp	Left-to-right
10	json_contains_all JSON_CONTAINS_ALL	JsonArrayOp	Left-to-right
10	array_contains_all ARRAY_CONTAINS_ALL	ArrayOp	Left-to-right
11	json_contains_any JSON_CONTAINS_ANY	JsonArrayOp	Left-to-right
11	array_contains_any ARRAY_CONTAINS_ANY	ArrayOp	Left-to-right
12	array_length ARRAY_LENGTH	ArrayOp	Left-to-right
13	&& and	BinaryLogicOp	Left-to-right
14	|| or	BinaryLogicOp	Left-to-right

Expressions are normally evaluated from left to right. Complex expressions are evaluated one at a time. The order in which the expressions are evaluated is determined by the precedence of the operators used.

If an expression contains two or more operators with the same precedence, the operator to the left is evaluated first.

When a lower precedence operation should be processed first, it should be enclosed within parentheses.

Parentheses can be nested within expressions. Innermost parenthetical expressions are evaluated first.

Usage

Samples of all available boolean expression usage in Milvus are listed as follows (int64 represents the scalar field that contains data of INT64 type, float represents the scalar field that contains data of floating-point type, and VARCHAR represents the scalar field that contains data of VARCHAR type):

1. CmpOp

"int64 > 0"

"0 < int64 < 400"

"500 <= int64 < 1000"

VARCHAR > "str1"

2. BinaryLogicalOp and parentheses

"(int64 > 0 && int64 < 400) or (int64 > 500 && int64 < 1000)"

3. TermExpr and UnaryLogicOp

"int64 not in [1, 2, 3]"

VARCHAR not in ["str1", "str2"]

4. TermExpr, BinaryLogicalOp, and CmpOp (on different fields)

"int64 in [1, 2, 3] and float != 2"

5. BinaryLogicalOp and CmpOp

"int64 == 0 || int64 == 1 || int64 == 2"

6. CmpOp and UnaryArithOp or BinaryArithOp

"200+300 < int64 <= 500+500"

7. MatchOp

VARCHAR like "prefix%"
VARCHAR like "%suffix"
VARCHAR like "%middle%"
VARCHAR like "_suffix"

8. JsonArrayOp

• JSON_CONTAINS(identifier, JsonExpr)

  If the JSON expression of a JSON_CONTAINS (the second argument) statement is a list, the identifier (the first argument) should be list of list. Otherwise, the statement always evaluates to False.

  # {"x": [1,2,3]}
  json_contains(x, 1) # ==> true
  json_contains(x, "a") # ==> false
      
  # {"x": [[1,2,3], [4,5,6], [7,8,9]]}
  json_contains(x, [1,2,3]) # ==> true
  json_contains(x, [3,2,1]) # ==> false
  

• JSON_CONTAINS_ALL(identifier, JsonExpr)

  The JSON expression in a JSON_CONTAINS_ALL statement should always be a list.

  # {"x": [1,2,3,4,5,7,8]}
  json_contains_all(x, [1,2,8]) # ==> true
  json_contains_all(x, [4,5,6]) # ==> false 6 is not exists
  

• JSON_CONTAINS_ANY(identifier, JsonExpr)

  The JSON expression in a JSON_CONTAINS_ANY statement should always be a list. Otherwise, it acts the same as JSON_CONTAINS.

  # {"x": [1,2,3,4,5,7,8]}
  json_contains_any(x, [1,2,8]) # ==> true
  json_contains_any(x, [4,5,6]) # ==> true
  json_contains_any(x, [6,9]) # ==> false
  

9. ArrayOp

• ARRAY_CONTAINS(identifier, ArrayExpr)

  If the array expression of an ARRAY_CONTAINS (the second argument) statement is a list, the identifier (the first argument) should be list of list. Otherwise, the statement always evaluates to False.

  # 'int_array': [1,2,3]
  array_contains(int_array, 1) # ==> true
  array_contains(int_array, "a") # ==> false
  

• ARRAY_CONTAINS_ALL(identifier, ArrayExpr)

  The array expression in an ARRAY_CONTAINS_ALL statement should always be a list.

  # "int_array": [1,2,3,4,5,7,8]
  array_contains_all(int_array, [1,2,8]) # ==> true
  array_contains_all(int_array, [4,5,6]) # ==> false 6 is not exists
  

• ARRAY_CONTAINS_ANY(identifier, ArrayExpr)

  The array expression in an ARRAY_CONTAINS_ANY statement should always be a list. Otherwise, it acts the same as ARRAY_CONTAINS.

  # "int_array": [1,2,3,4,5,7,8]
  array_contains_any(int_array, [1,2,8]) # ==> true
  array_contains_any(int_array, [4,5,6]) # ==> true
  array_contains_any(int_array, [6,9]) # ==> false
  

• ARRAY_LENGTH(identifier)

  Check the number of elements in an array.

  # "int_array": [1,2,3,4,5,7,8]
  array_length(int_array) # ==> 7
  

What’s next

Now that you know how bitsets work in Milvus, you might also want to:

• Learn how to conduct a Hybrid Search.
• Learn how to use strings to filter your search results.
• Learn how to use dynamic fields in building boolean expressions.