homework-jianmu/docs/user_manual/Small_Materialized_Aggrates.md

Small Materialized Aggragates

SMA (Small Materialized Aggrates) is used to speed up the query process on materialized data cube in TDengine. TDengine 3.0 gives more flexibility on the SMA configurations.

There are two kinds of SMA in TDengine:

1. Block-wise SMA
2. Time-range-wise SMA

Block-wise SMA

Block-wise SMA is created by default when the data are committed. Since time-series data are saved as block data in files, a corresponding SMA is create when the data block is written. The default block-wise SMA includes:

1. sum(*)
2. max(*)
3. min(*)

By default, the system will create SMA for each column except those columns with type binary and nchar. However, users can change the behavior by the keyword NOSMA to disable the SMA for a certain column like below:

# create a super table with the SMA on column b disabled
create table st (ts timestamp, a int, b int NOSMA, c double) tags (tg1 binary(10), tg2 int);

Time-range-wise SMA

In addition to the default block-wise SMA, users can create their own SMAs ondemand. Below is an example to create a SMA.

# create a SMA every 10 minutes with SMA of sum, max and min
create sma_indx sma_10min on st (sum(*), max(*), min(*), twa(*)) interval(10m);

Users can also drop a time-range-wise SMA like below:

# drop the sma index
drop sma_index sma_5min on st;

NOTE: Creating an SMA index is a heavy operation which may take a long time and block the write operation. So create the time-range-wise SMA when creating the table or when there are not too much data.