Adding decimal to documentation

documentation/concept/decimal.md

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+---
+title: Decimal
+description:
+  This document describes how to work with the decimal data type in QuestDB,
+  including precision and scale, literal syntax, arithmetic operations, and best
+  practices for financial and scientific calculations.
+---
+
+QuestDB provides a `decimal` data type for exact numeric calculations, useful
+for financial computations, scientific measurements, and any scenario where
+precision matters. This page explains how to use decimals effectively, including
+syntax, operations, and performance considerations.
+
+## What are decimals?
+
+Decimals are fixed-point numbers that maintain exact precision during arithmetic
+operations. Unlike floating-point types (`float` and `double`), decimals avoid
+rounding errors by storing numbers as scaled integers internally. This makes
+them ideal for monetary calculations where accuracy is critical.
+
+## Decimal type in QuestDB
+
+QuestDB implements decimals with the syntax `DECIMAL(precision, scale)`:
+
+- **Precision**: Total number of significant digits (1-76)
+- **Scale**: Number of digits after the decimal point (0-precision)
+
+For example, `DECIMAL(10, 2)` can store values from -99,999,999.99 to
+99,999,999.99.
+
+If neither the precision and scale are provided, the type defaults to a
+precision of 18 and a scale of 3.
+
+### Storage
+
+QuestDB automatically selects the optimal storage size based on the decimal's
+precision:
+
+| Precision    | Storage Size | Internal Type |
+| ------------ | ------------ | ------------- |
+| 1-2 digits   | 1 byte       | DECIMAL8      |
+| 3-4 digits   | 2 bytes      | DECIMAL16     |
+| 5-9 digits   | 4 bytes      | DECIMAL32     |
+| 10-18 digits | 8 bytes      | DECIMAL64     |
+| 19-38 digits | 16 bytes     | DECIMAL128    |
+| 39-76 digits | 32 bytes     | DECIMAL256    |
+
+## Decimal literals
+
+QuestDB requires the `m` suffix to distinguish decimal literals from
+floating-point numbers:
+
+```questdb-sql
+-- Decimal literals use the 'm' suffix
+SELECT 123.45m;        -- Decimal value 123.45
+SELECT 0.001m;         -- Decimal value 0.001
+SELECT 1000000.00m;    -- Decimal value 1,000,000.00
+
+-- Without 'm' suffix, numbers are treated as double
+SELECT 123.45;         -- Double value (floating-point)
+```
+
+:::important
+
+Always use the `m` suffix for decimal literals. QuestDB does not implicitly
+convert doubles to decimals to prevent unintended precision loss.
+
+:::
+
+## Creating tables with decimals
+
+Define decimal columns by specifying precision and scale:
+
+```questdb-sql
+CREATE TABLE transactions (
+    id LONG,
+    amount DECIMAL(14, 2),      -- Up to 999,999,999,999.99
+    tax_rate DECIMAL(5, 4),      -- Up to 9.9999 (e.g., 0.0875 for 8.75%)
+    quantity DECIMAL(10, 3),     -- Up to 9,999,999.999
+    timestamp TIMESTAMP
+) timestamp(timestamp);
+```
+
+## Working with decimals
+
+### Basic arithmetic
+
+Decimal arithmetic maintains precision automatically:
+
+```questdb-sql
+-- Insert transaction data
+INSERT INTO transactions VALUES
+    (1, 99.99m, 0.0875m, 2.500m, now()),
+    (2, 150.00m, 0.0625m, 1.750m, now()),
+    (3, 1250.50m, 0.0875m, 10.000m, now());
+
+-- Arithmetic operations maintain precision
+SELECT
+    amount,
+    amount * tax_rate AS tax_amount,
+    amount + (amount * tax_rate) AS total,
+    amount * quantity AS extended_amount
+FROM transactions;
+```
+
+### Precision and scale in operations
+
+QuestDB automatically determines the result precision and scale for decimal
+operations based on the operands:
+
+#### Addition and subtraction
+
+- **Scale**: Maximum scale of the operands
+- **Precision**: Maximum precision of the operands (scaled) + 1
+
+```questdb-sql
+-- Addition with different scales
+SELECT 10.5m + 1.234m;  -- scale: max(1, 3) = 3, Result: 11.734
+
+-- Adding DECIMAL(10,2) + DECIMAL(8,2) → DECIMAL(11,2)
+SELECT 99999999.99m + 999999.99m;  -- Result has precision 11, scale 2
+```
+
+The additional precision digit allows the result to accommodate potential
+overflow (e.g., 99.9 + 99.9 = 199.8 requires 4 digits instead of 3).
+
+#### Multiplication
+
+- **Scale**: Sum of the scales of both operands
+- **Precision**: Sum of the precision of both operands
+
+```questdb-sql
+-- Multiplication adds scales
+SELECT 10.50m * 1.25m;  -- scale: 2 + 2 = 4, Result: 13.1250
+
+-- DECIMAL(5,2) * DECIMAL(4,2) → DECIMAL(9,4)
+SELECT 100.50m * 12.34m;  -- Result: 1240.1700
+```
+
+#### Division
+
+- **Scale**: Maximum scale of the operands
+
+```questdb-sql
+-- Division uses maximum scale
+SELECT 10.50m / 2.0m;  -- scale: max(2, 1) = 2, Result: 5.25
+
+-- Division may truncate beyond the scale
+SELECT 10.00m / 3.00m;  -- Result: 3.33 (limited to scale 2)
+```
+
+### Comparison operations
+
+Decimals support all standard comparison operators:
+
+```questdb-sql
+-- Find high-value transactions
+SELECT * FROM transactions WHERE amount > 1000.00m;
+
+-- Find specific tax rates
+SELECT * FROM transactions WHERE tax_rate = 0.0875m;
+
+-- Range queries
+SELECT * FROM transactions WHERE amount BETWEEN 100.00m AND 500.00m;
+```
+
+## Type casting
+
+### Explicit casting
+
+Convert between numeric types using `CAST`:
+
+```questdb-sql
+-- From integer to decimal
+SELECT CAST(100 AS DECIMAL(10, 2));  -- Result: 100.00
+
+-- From double to decimal (use with caution - may lose precision)
+SELECT CAST(123.456789 AS DECIMAL(8, 3));  -- Result: 123.457
+
+-- From decimal to other types
+SELECT CAST(99.99m AS INT);    -- Result: 99 (truncate)
+SELECT CAST(99.99m AS DOUBLE);  -- Result: 99.99 (as floating-point)
+```
+
+### Important casting rules
+
+- **No implicit conversion from double/float**: Must use explicit `CAST` or
+  decimal literals
+- **Integer to decimal**: Safe, no precision loss, the decimals have a scale of
+  0
+- **Double to decimal**: May lose precision due to floating-point representation
+- **Between decimal types**: Automatic when precision/scale allows
+
+## Considerations
+
+### Advantages
+
+- **Exact results**: Perfect for financial calculations and accounting
+- **Predictable behavior**: No surprising rounding errors
+- **Regulatory compliance**: Meets requirements for exact monetary calculations
+
+### Trade-offs
+
+- **Slower than floating-point**: Typically slower than `double` operations
+- **More storage**: May use more space than `float` for equivalent range
+- **Complex operations**: Division have overhead
+
+### Performance tips
+
+- **Use appropriate precision**: Don't over-specify precision beyond your needs
+- **Keep precision ≤ 18 when possible**: DECIMAL64 operations are faster than
+  DECIMAL128/256
+
+## Common use cases
+
+### Financial calculations
+
+```questdb-sql
+-- Portfolio valuation with exact arithmetic
+CREATE TABLE portfolio (
+    symbol SYMBOL,
+    shares DECIMAL(12, 4),     -- Fractional shares supported
+    price DECIMAL(10, 2),       -- Stock price
+    commission DECIMAL(7, 2),   -- Trading fees
+    timestamp TIMESTAMP
+) timestamp(timestamp);
+
+-- Calculate exact portfolio value
+SELECT
+    symbol,
+    shares,
+    price,
+    shares * price AS position_value,
+    shares * price - commission AS net_value,
+    sum(shares * price) OVER () AS total_portfolio_value
+FROM portfolio
+WHERE timestamp = now();
+```
+
+### Cryptocurrency trading
+
+```questdb-sql
+-- ETH trading with high precision (18 decimals like wei)
+CREATE TABLE crypto_trades (
+    trade_id LONG,
+    pair SYMBOL,
+    eth_amount DECIMAL(28, 18),      -- ETH with full wei precision
+    usdt_price DECIMAL(12, 2),        -- USDT price per ETH
+    fee_rate DECIMAL(5, 4),           -- Trading fee (e.g., 0.001 for 0.1%)
+    gas_fee_eth DECIMAL(18, 18),      -- Gas fee in ETH
+    timestamp TIMESTAMP
+) timestamp(timestamp);
+
+-- Calculate trade values with exact precision
+SELECT
+    trade_id,
+    eth_amount,
+    usdt_price,
+    eth_amount * usdt_price AS trade_value_usdt,
+    eth_amount * usdt_price * fee_rate AS fee_usdt,
+    eth_amount * usdt_price * (1.0m - fee_rate) AS net_value_usdt,
+    eth_amount - gas_fee_eth AS net_eth_received
+FROM crypto_trades;
+```
+
+### Scientific measurements
+
+```questdb-sql
+-- High-precision sensor data
+CREATE TABLE sensor_readings (
+    sensor_id SYMBOL,
+    measurement DECIMAL(20, 10),  -- 10 decimal places of precision
+    calibration_factor DECIMAL(6, 5),
+    timestamp TIMESTAMP
+) timestamp(timestamp);
+
+-- Apply calibration with exact arithmetic
+SELECT
+    sensor_id,
+    measurement,
+    measurement * calibration_factor AS calibrated_value,
+    avg(measurement) OVER (PARTITION BY sensor_id) AS avg_reading
+FROM sensor_readings
+SAMPLE BY 1h;
+```
+
+## Best practices
+
+### When to use decimals
+
+**Use decimals for:**
+
+- Financial data (prices, amounts, exchange rates)
+- Accounting calculations
+- Scientific measurements requiring exact precision
+- Regulatory compliance scenarios
+- Any calculation where rounding errors are unacceptable
+
+**Avoid decimals for:**
+
+- Scientific calculations requiring extensive math functions
+- Performance-critical analytics on large datasets
+- Approximate values where precision isn't critical
+- Coordinates or measurements where float precision suffices
+
+### Design guidelines
+
+1. **Choose appropriate precision and scale**
+
+   ```questdb-sql
+   -- Good: Matches business requirements
+   CREATE TABLE prices (
+       amount DECIMAL(10, 2)  -- Cents precision for USD
+   );
+
+   -- Avoid: Excessive precision
+   CREATE TABLE prices (
+       amount DECIMAL(30, 15)  -- Unnecessary for most use cases
+   );
+   ```
+
+2. **Use the 'm' suffix consistently**
+
+   ```questdb-sql
+   -- Good: Clear decimal literals
+   INSERT INTO prices VALUES (99.99m);
+
+   -- Error: Missing 'm' suffix
+   INSERT INTO prices VALUES (99.99);  -- Treated as double, will fail
+   ```
+
+3. **Explicit casting when mixing types**
+
+   ```questdb-sql
+   -- Good: Explicit cast
+   SELECT amount + CAST(10 AS DECIMAL(10, 2)) FROM prices;
+
+   -- Good: Use decimal literal
+   SELECT amount + 10.00m FROM prices;
+   ```