如何检查已有数据表是否存在指定时间戳及QuestDB中已有表指定时间戳列的查询方法

Hey there! Let's tackle your two QuestDB-related questions clearly and practically:

1. How to check if a specific timestamp value exists in a table #

If you already know which column holds timestamp values, this is straightforward with a SELECT EXISTS query—it’s efficient because it stops searching as soon as it finds a match:

SELECT EXISTS(
  SELECT 1 
  FROM your_table_name 
  WHERE your_timestamp_column = '2024-05-20T14:30:00Z' -- Use ISO 8601 format, or epoch ms/sec
) AS timestamp_exists;

• Replace your_table_name and your_timestamp_column with your actual table and column names.
• QuestDB supports ISO 8601 strings (like the example), epoch milliseconds (e.g., 1716229800000), or epoch seconds (e.g., 1716229800) for timestamp comparisons.

If you don’t know which column might hold the timestamp, you’ll first need to identify timestamp-type columns (see question 2 below), then run checks against each one.

2. How to check if a QuestDB table has a timestamp column (and identify which one) #

QuestDB has two key scenarios here: checking for any timestamp-type column, or checking for the designated timestamp column (a QuestDB-specific feature used for time partitioning).

Option A: List all timestamp-type columns in a table #

Use the information_schema.columns system table to query column types:

SELECT column_name 
FROM information_schema.columns 
WHERE table_name = 'your_table_name' 
  AND data_type IN ('TIMESTAMP', 'TIMESTAMP WITH TIME ZONE');

This will return every column in your table that’s of timestamp type.

Option B: Find the table’s designated timestamp column #

If you’re looking for the column configured as the table’s time partition key (used for retention policies and time-based queries), query the system.tables table:

SELECT designated_timestamp 
FROM system.tables 
WHERE name = 'your_table_name';

• If the table has a designated timestamp, this returns the column name; if not, it returns NULL.

Bonus: Find which column contains a specific timestamp value #

If you want to locate exactly which column holds your target timestamp value, use a union of limited checks (note: this is less efficient for large tables, so use sparingly):

SELECT 'column1' AS matching_column FROM your_table WHERE column1 = '2024-05-20T14:30:00Z' LIMIT 1
UNION ALL
SELECT 'column2' AS matching_column FROM your_table WHERE column2 = '2024-05-20T14:30:00Z' LIMIT 1
-- Add more timestamp columns as needed

This will return all columns that contain your target timestamp value.

内容的提问来源于stack exchange，提问作者Newskooler